Gilbert 
Light  Experiments 

FOR  BOYS 

BY 

CARLETON  JOHN  LYNDE,  Ph.D. 

Professor  of  Physics 
MacDONALD  COLLEGE 
QUEBEC  PROVINCE,  CANADA 

Under  the  Direction  of 

ALFRED  C.  GILBERT 

Yale  University  •  1909 
PUBLISHED  BY 

THE  A.  C.  GILBERT  COMPANY 

NEW  HAVEN,  CONN. 


New  York       Chicago       San  Francisco       Toronto  London 


Copyrighted,  1920,  by  A.  C.  Gilbert 
New  Haven,  Conn. 


THE  GETTY  CENTER 
LIBRARY 


Foreword 


The  things  that  are  the  most  common  are  often  the  ones  that 
people  know  the  least  about.  This  is  true  of  light.  Few  boys 
have  even  taken  the  trouble  to  get  the  essential  facts  about  this 
subject.  You  can  realize  how  important  it  is  when  you  are 
told  that  without  the  sun  —  our  main  source  of  light  —  there 
would  be  no  life  at  all.  There  would  be  no  growth  of  plants, 
that's  sure.  I  know  as  a  boy  my  curiosity  was  always  prompt- 
ing me  to  ask  questions.  I  wanted  to  know  the  facts  and  reasons 
for  everything.   I  believe  most  boys  are  the  same  in  that  respect. 

This  book  has  been  written  so  that  you  can  get  information 
first  hand  on  a  mighty  interesting  subject.  It  is  in  plain  language, 
which  you  can  readily  understand,  and  a  study  of  it  will  soon 
make  you  familiar  with  great  scientists,  who  have  made  laws 
of  great  importance.  Their  discoveries  make  possible  the  use 
of  a  number  of  instruments  that  you  know  so  well.  The  telescope 
- —  the  opera  glass  —  the  moving-picture  machine  —  are  just  a  few 
which  can  be  mentioned,  and  you  know  how  necessary  they  are  in 
the  world  to-day. 

In  learning  facts  about  light  and  some  of  the  inventions  made 
by  great  men  you  will  get  a  knowledge  that  very  few  boys 
have.  You  will  be  able  to  talk  very  interestingly  about  light 
and  it  will  be  easy  for  you  to  explain  a  great  many  questions 
that  come  up  in  connection  with  it.  Best  of  all,  you  will  have 
a  whole  pile  of  fun  and  at  the  same  time  get  a  good  understand- 
ing of  the  fundamentals  of  the  science  of  light. 


Sincerely  yours, 


Digitized  by  the  Internet  Archive 
in  2013 


http://archive.org/details/gilbertlightexpeOOIynd 


GILBERT  LIGHT  EXPERIMENTS 


FUN  WITH  BRIGHT  SUNLIGHT 

Experiment  No.  1.  To  obtain  more 
than  one  million  miles  of  sunshine.  Go 
outside  with  your  watch  in  your  hand 
and  stand  in  the  sunlight  (Fig.  1)  for  just 
six  seconds.  In  that  short  six  seconds 
you  will  have  received  more  than  1,000,- 
000  miles  of  sunlight.  Light  travels  at 
the  enormous  velocity  of  186,000  miles 
per  second,  and  therefore  in  six  seconds 
you  receive  on  your  body  186,000  X  6  = 
1,116,000  miles  of  sunlight. 

Experiment  No.  2.  To  receive  over 
two  thousand  million  million  light  waves. 
Hold  your  hand  in  a  beam  of  sunlight 
(Fig.  2)  for  six  seconds  and  then  with- 
draw it. 


Fig.  1.    You  receive 
1,000,000  miles  of  sunshine 


Fig.  2.  You  receive  2,000,- 
000,000,000  light  waves 


You  will  show  soon  that  white  light 
is  a  mixture  of  lights  of  all  colors.  Now 
the  red  light  waves  in  the  sunlight  fall 
on  your  hand  at  the  rate  of  390  million 
million  per  second  and  in  six  seconds 
390  X  6  =  2340  million  million  fell  on 
your  hand.  Your  hand  received  many 
more  than  this  because  waves  of  all  colors 
fell  on  it,  and  of  violet  light  alone  it 
received  twice  the  above  number. 

Repeat  this  experiment  with  a  candle 
flame,  or  oil  lamp,  or  electric  light.  In 
each  case  your  hand  receives  more  than 
two  thousand  million  million  light  waves 
in  the  six  seconds. 


6 


GILBERT  BOY  ENGINEERING 


TO  MAKE  YOUR  DARK  ROOM 

Boys,  you  are  going  to  make 
many  experiments  with  a  beam  of 
sunlight  let  into  a  darkened  room, 
so  prepare  for  them  now  thor- 
oughly, as  follows : 

Select  in  your  home  a  room 
with  only  one  window,  facing  the 
south  or  east  or  west.  Now  cover 
this  window  so  that  no  light  can 
enter  the  room  except  through  a 
small  slit.    The  best  way  to  do 


Fig.  3.    Shutter  to  darken  room 

this  is  to  make  a  solid  wooden  shut- 
ter, with  tar  paper  on  one  side,  to 
cover  the  whole  window  and  make 
a  slit  in  it  three  inches  wide  and 
two  inches  high.  If  the  window  is 
too  large  for  this,  make  a  wooden 
shutter  (Fig.  3)  for  the  lower  part 
and  cover  the  upper  part  with  black 
cloth,  black  paper,  a  quilt,  a  heavy 
blanket  (Fig.  4),  or  anything  that 
will  shut  out  all  light.  Make  the 
slit  in  the  wooden  part  because  you 
will  want  to  change  the  size  and 


Fig.  4.  Shutter  in  window  with 
other  side  showing  and  with  card- 
board tacked  over  slit 


GILBERT  LIGHT  EXPERIMENTS 


7 


shape  of  the  slit.  To  clo  this  you  will 
make  slits  of  the  right  kind  in  black 
tar  paper  or  cardboard  and  then  tack 
these  over  the  slit  in  the  wooden  shut- 
ter. Make  the  shutter  so  that  you  can 
take  it  down  and  put  it  up  quickly,  be- 
cause you  will  want  to  experiment 
many  times  and  your  mother  will, 
probably,  not  want  to  leave  the  win- 
dow permanently  darkened. 


TO  MAKE  A  DARK  BOX 


Fig.  5.   Your  dark  box 


If  you  cannot  make  a  dark  room, 
you  can  make  a  dark  box  (Fig.  5),  as  follows  :  Take  a  packing  box 
V/2  X  V/2'  X  2'  or  larger,  bore  a  hole  2T/2  inches  in  diameter  in 
the  center  of  one  end,  cover  the  open  top  with  a  piece  of  dark 
light-proof  cloth,  4'  X  3',  tacked  to  the  ends  and  one  side.  Plait 
this  along  the  side  to  leave  room  for  your  head  and  shoulders. 
Make  the  box  light-proof,  turn  it  on  its  side  with  sunlight  enter- 
ing the  slit,  and  you  are  ready  to  make  your  experiments. 

It  improves  the  box  to  paint  it  black  on  the  inside.  When 
you  need  a  slit,  cut  it  in  cardboard  and  tack  the  cardboard  over 
the  2^-inch  hole.  This  hole  will  just  take  your  large  ring  lens 
holder  when  you  experiment  with  lenses. 

A  dark  room  is  more  fun  than  a  dark  box,  and  the  directions 
in  this  book  assume  that  you  have  made  one.  You  will  find,  how- 
ever, that  you  can  make  most  of  the  dark-room  experiments  in 
the  dark  box.    It  is  an  excellent  idea  to  have  both. 

Experiment  No.  3.  To  show  that  you  cannot  see  a  beam  of 
light  unless  it  falls  on  some  object  or  directly  on  your  eyes. 
Darken  the  room  on  a  day  when  the  sun  is  shining  in  the  win- 
dow. Leave  the  room  for  five  minutes  to  let  the  dust  settle  and 
then  return.   Do  you  find  that  you  cannot  see  the  beam  between 


8 


GILBERT  BOY  ENGINEERING 


the  slit  and  a 
screen  or  the  wall 
or  floor? 

Make  a  dust 
near  the  beam  by 
shaking  your  coat 
or  a  carpet.  Do 
you  now  appear  to 
see  the  beam  (Fig. 
6)?  You  do,  be- 
cause the  dust  par- 


rig.  6.   Dust  makes  the  beam  visible  tides  reflect  light 

to  your  eyes. 

On  a  clear  night  you  cannot  see  the  beam  from  the  headlight 
of  a  locomotive ;  but  when  there  is  mist,  rain,  or  snow,  you  appear 
to  see  it  because  particles  of  these  reflect  light  to  your  eyes.  In 
either  case  you  can  see  any  object  the  beam  falls  on  because  it  re- 


Fig.  7.    A  beautiful  inverted  picture 
From  Appleton's  School  Physics,  published  by  the  American  Booh  Oo. 


GILBERT  LIGHT  EXPERIMENTS 


9 


fleets  light  to  your  eyes.  You 
can  also  see  the  light  if  it  falls 
directly  on  your  eyes. 

Experiment  No.  4.  To  show 
that  light  travels  in  a  straight 
line.  Make  a  dust  near  the 
beam.  Does  the  light  travel  in 
a  straight  line  from  the  slit  to 
the  wall  or  floor  or  paper 
screen? 

Experiment  No.  5.  To  get    Fig>  8>   You  see  a  beautifui  picture  in 

a    picture    of    all    OUt-of-doorS.  natural  colors 

Punch  a  nail  hole  in  a  piece  of  black  paper  or  cardboard,  tack  the 
paper  or  cardboard  over  the  slit  in  your  darkened  room,  and  hold 
a  sheet  of  tissue  paper  about  one  foot  from  the  hole.  Do  you  find 
on  the  paper  a  picture  (Figs.  7  and  8)  of  the  whole  view  out-of- 
doors  opposite  the  hole?  Is  the  picture  inverted  and  in  natural 
colors?    Can  you  see  men,  horses,  and  automobiles  moving? 

This  is  a  fascinating  experiment,  and  it  shows  best  when  the 
sun  is  shining  on  the  landscape  and  not  on  the  window. 

The  picture  is  inverted  because  light  travels  in  straight  lines. 
The  sunlight  which  falls  on  any  part  of  a  cloud,  for  example,  is 
reflected  in  all  directions  in  straight  lines,  and  a  very  small  part  of 
this  light  passes  through  the  hole  to  the  bottom  of  the  tissue  paper. 
Also  the  sunlight  which  falls  on  any  object  on  the  ground  is  re- 
flected in  all  directions  in  straight  lines,  and  a  very  small  part  of 
this  passes  through  the  hole  to  the  top  of  the  tissue  paper,  and  so 
on.  That  is,  the  picture  is  inverted  because  light  travels  in  a 
straight  line  from  each  object  through  the  hole  to  the  paper. 

The  picture  is  in  natural  colors  because  each  object  reflects 
light  of  its  own  color  and  absorbs  the  remainder.  That  is,  the 
blue  sky,  green  grass,  and  red  bricks  reflect  blue,  green,  and  red 
light  respectively,  and  so  on. 


10  GILBERT  BOY  ENGINEERING 


Move  the  paper  farther  from  the  hole.  Is  the  picture  larger 
but  dimmer?  It  is  larger  because  the  rays  from  different  parts  of 
the  view  cross  at  the  hole  and  diverge  afterward.  It  is  dimmer 
because  only  a  certain  amount  of  light  passes  through  the  hole, 
and  it  covers  a  larger  area  the  farther  the  paper  is  from  the  hole. 
Punch  a  second  nail  hole  two  inches  from  the  first.  Do  you  nowget 
two  pictures?  Do  they  blur  where  they  overlap?  Punch  many  holes. 
Do  you  get  as  many  pictures,  but  do  they  blur  more  and  more? 

Open  the  slit  to  its  full  size.  Do  you  find  that  there  is  no 
picture  at  all,  but  just  white  light?  There  is  no  picture  because 
light  from  all  parts  of  the  view  falls  on  all  parts  of  the  picture 
and  the  combination  of  all  colors  produces  white  light. 

Make  a  hole  the  size  of  a  lead  pencil  in  a  new  piece  of  black 
paper  and  tack  the  paper  over  the  slit.  Do  you  get  a  brighter  pic- 
ture, but  is  it  more  indistinct  than  with  the  nail  hole  ?  Remove  the 
tissue  paper.  Is  there  a  picture  on  the  opposite  wall  ?  This  will  show 
only  if  the  sun  is  shining  brightly  on  the  view,  and  if  your  room  is 
completely  dark  except  for  the  light  which  passes  through  the  hole. 

Experiment  No.  6.  To  get  a  picture  of  the  sun.  Allow  a  beam 
of  sunlight  to  pass  through  a  nail  hole  into  your  darkened  room, 

catch  it  on  a  piece  of 
paper  and  move  the 
paper  back  and  forth. 
Is  the  image  round 
(Fig.  9)  and  is  it  larger 
the  farther  the  paper  is 
from  the  hole? 

The  image  is  round 
because  the  sun  is 
round.  It  increases  in 
size  because  the  light 
rays  from  the  sun  trav- 
el in  straight  lines  and 


GILBERT  LIGHT  EXPERIMENTS  11 


cross  at  the  hole.  The 
upper  side  of  the  sun 
sends  out  light  in  all 
directions  in  straight 
lines ;  a  very  small  part 
of  this  passes  through 
the  hole  (H,  Fig.  11) 
and  makes  an  image  of 
itself  at  the  bottom  of 
the  picture ;  similarly 
light  from  the  lower 
side  of  the  sun  makes  Fig  10   you  gee  two  images  of  the  sun 

an  image  of  itself  at  the 

top  of  the  picture,  and  so  on.  Reflect  the  light  to  the  farthest  part 
of  the  room  by  means  of  a  mirror.  Is  the  image  larger  the  farther 
it  is  from  the  hole?  Punch  two  holes.  Do  you  get  two  images 
(Fig.  10)  ?  Punch  many  holes.  Do  you  get  many  images,  but 
do  they  overlap? 

Open  the  slit  entirely.  Do  you  get  only  a  bright  spot  in  the 
shape  of  the  slit?  This  spot  is  made  up  of  many,  many  round 
images,  and  you  will  notice  that  the  edges  and  corners  are  some- 
what blurred  and  not  sharp. 

Take  a  new  piece  of  black  paper,  make  a  triangular  hole  one- 
quarter  inch  on  a  side,  tack  it  over  the  slit  and  get  an  image  of  the 
sun  at  one  inch  from  the  hole,  then  at  greater  and  greater  dis- 
tances.  Is  the  image  at  first  triangular  and  does  it  become  more 

and  more  blurred  at  the 
sides  and  corners  until 
finally  it  is  round?  The 
image  is  made  up  of 
many  small,  round  im- 
ages of  the  sun,  and 

Fig.  11.    The  rays  cross  at  the  nail  hole  when    these    are  large 


12 


GILBERT  BOY  ENGINEERING 


compared  to  the  size  of  the  hole  they 
overlap  and  produce  a  round  image. 

Repeat  with  a  square  hole  one- 
quarter  inch  on  each  side.  Are  the  re- 
sults similar? 

You  have  probably  noticed  that  sun- 
light produces  round  images  of  the  sun 
when  it  passes  through  any  small  open- 
ing; for  example,  in  a  shutter  or  blind, 
between  the  leaves  of  trees,  and  so  on. 
The  explanation  is  that  given  above. 

Experiment  No.  7.  To  make  a 
pinhole  camera.  Make  a  nail  hole 
in  the  middle  of  the  bottom  of  a 
cardboard  box,  cover  the  open  top  with 
a  piece  of 


Fig.  12.  The  open  side  of  the 
box  is  covered  with  tissue  paper 


tissue  paper 
(Fig.  12), 
hold  the  hole 

toward  a  brightly  lighted  landscape, 
cover  your  head  and  the  tissue  paper 
with  a  black  cloth  or  blanket  to  shut  out 
all  the  light  (Fig.  13),  and  look  at  the 
tissue  paper.  Do  you  see  a  beautiful 
image  of  the  landscape  inverted  and  in 
natural  colors? 

This  is  a  beautiful  experiment  and 
it  is  explained  as  above. 

You  can  actually  make  pictures 
through  a  pinhole,  as  follows  :  Remove 
the  lens  from  a  camera,  cover  the  open- 
ing with  heavy  tin  foil  and  pierce  the 
foil  with  a  pin.    Now  to  take  the  pic- 


see  a  beautiful 
picture  on  the  paper 


GILBERT  LIGHT  EXPERIMENTS  13 


Fig.  14.   Spherical  waves  and 
straight  rays 


ture,  cover  the  pinhole,  arrange  the 
plate  or  film  in  position,  uncover  the 
pinhole  for  a  short  time,  cover  it,  and 
develop  your  negative  as  usual. 

SOMETHING  ABOUT  LIGHT 

Now,  boys,  before  we  go  any  further 
let  us  get  some  clear  ideas  about  light. 

Light  is  that  which  produces  on  the 
eyes  the  sensation  of  sight. 

Medium.     A  medium  is  anything 
through  which  light  travels ;  for  example,  air,  water,  glass,  and 
the  ether. 

Ether.  The  ether  is  supposed  to  be  a  very  thin  and  elastic 
medium  which  fills  all  space,  not  only  the  space  between  the 
planets,  but  also  the  space  between  the  smallest  particles  (mole- 
cules) of  solids,  liquids,  and  gases. 

How  Light  is  Produced.  Light  is  produced  by  the  vibration 
of  very  hot  particles  of  matter. 

For  many  reasons,  scientists  believe  that  the  smallest  particles 
of  all  substances  are  vibrating,  that  is,  moving  back  and  forth  in 
all  directions,  all  the  time,  and  that  the  hotter  they  are  the  faster 
they  vibrate.  Now  in  the  flame  of  a  candle,  oil  lamp,  or  gas  jet 
there  are  particles  of  unburned  carbon  which  are  very  hot  and  are, 
therefore,  vibrating  rapidly.  These  vibrating  particles  set  the 
ether  in  the  flame  in  vibration,  and  these  vibrations  spread  out 

wwwwww  wwww 


Fig.  15.    A  beam  of  light.    W  =  parallel  waves.    R  =  parallel  rays 


14  GILBERT  BOY  ENGINEERING 


in  all  directions  in  the  form  of  spherical  waves  in  the  ether.  These 
ether  waves  are  light  waves  or  heat  waves. 

Similarly  the  light  of  an  incandescent  electric  arc  light  or  of 
the  sun  is  produced  by  rapidly  vibrating  hot  particles  of  matter. 

Note.  Heat  waves  are  longer  than  light  waves  and  do  not 
produce  the  sensation  of  sight,  but  they  are  similar  to  light  waves 
in  all  other  respects. 

Waves  and  Rays.  If  the  dot  in  the  center  of  Fig.  14  is  a  rap- 
idly vibrating  particle,  the  circles  about  it  will  give  the  position  of 


Fig.  16.    The  lens  produces  a  converging  pencil  and  a  diverging  pencil 
W  =  waves.   R  =  rays 


its  light  waves  after  equal  intervals  of  time,  but  the  light  waves 
are  spherical  instead  of  circular.  The  straight  arrows  drawn  from 
the  center  represent  light  rays.  They  give  the  path  along  which 
the  light  is  traveling  in  all  directions  from  the  center.  The  light 
waves  are  real  and  produce  the  sensation  of  sight ;  the  rays  are  not 
real,  they  are  imaginary,  straight  lines  which  give  the  direction 
of  the  light  and  they  are  always  at  right  angles  to  the  waves. 

Parallel  Waves  and  Rays.  The  waves  from  the  dot  are  larger 
the  farther  they  are  from  the  center,  and  when  they  are  one  hun- 
dred yards  or  a  mile  from  the  center  they  are  very  large  indeed. 
If  your  eye  receives  light  from  any  such  distant  point  the  small 
part  of  the  waves  which  enter  it  are  nearly  parallel  straight  lines, 


GILBERT  LIGHT  EXPERIMENTS  15 


and  since  the  rays  are  always  at  right 
angles  to  the  waves  they  are  also  nearly 
parallel.  This  is  particularly  true  if  the 
distant  point  is  the  sun,  at  a  distance  of 
ninety  million  miles.  Parallel  waves  and 
rays  then  are  those  from  a  distant  source. 
Beam.    Pencil.    A  beam  (Fig.  15) 

is  a  group  of  parallel  waves  and  rays.    A  Fig.  17.  The  candle  sends  rays 

•1  /t-«      -  n\  •  r  1  in  all  directions 

pencil  (Fig.  16)  IS  a  group  Of  waves  and  From  Appleton's  School  Phys- 
,  .  ,  '  .   ,  ics.  published  by  the  American 

rays  which  converge  at  a  point  or  diverge  Book  Co. 

from  it.    The  eyes  (Fig.  17)  are  receiving  diverging  pencils  of 

light  from  the  candle  which  is  sending  out  light  in  all  directions. 

Luminous  and  Non-luminous  Bodies.  Luminous  bodies  are 
those  which  give  out  light,  such  as  the  sun,  electric  light,  gas  jet, 
oil  lamp,  candle,  and  match.  Non-luminous  bodies  are  those 
which  do  not  give  out  light,  and  which  can  be  seen  only  by 
means  of  light  from  luminous  bodies. 

Transparent,  Translucent,  and  Opaque  Bodies.  Bodies  which 
you  can  see  through  are  called  transparent;  such  as  air,  water, 
and  glass.  Bodies  which  let  light  through,  but  which  you  cannot 
see  through,  are  called  translucent;  as  paper,  ground  glass,  and 
cotton  cloth.     Bodies  which  do  not  transmit  light  are  called 

opaque;  for  example,  wood, 
brick,  and  metal. 

No  substance  is  entirely 
opaque;  for  example,  even 
metals  let  through  some  light 
when  they  are  in  very  thin 
sheets. 

Straight  Lines.  Light  al- 
ways travels  in  straight  lines 
from  its  source  until  it  falls 
Fig.  18.   Unburned  carbon  in  a  flame  on  some  object  or   Until  it 


16 


GILBERT  BOY  ENGINEERING 


passes  into  another  me 
dium.  It  then  changes  I 
direction  but  again  | 
travels  in  straight  lines  I 
from  the  object  or  in  j 
the  new  medium. 

Experiment  No.  8. 
Carbon  particles  in  a 
flame.  Hold  a  saucer 
in  a  candle  flame  (Fig. 
18).  Is  it  blackened? 
The  blackening  is  caused  by  the  unburned  carbon  particles. 

Experiment  No.  9.  Water  waves.  Look  along  the  surface  of 
water  in  a  pan  and  dip  a  pencil  in  and  out.  Do  you  observe 
circular  waves  (Fig.  19)  ?  Throw  a  stone  into  a  still  pond  or 
lake.  Do  you  see  circular  waves?  These  illustrate  light  waves, 
but  the  light  waves  are  spherical. 


Fig.  19.   Circular  waves  on  water 


FUN  AT  NIGHT 

Experiment  No.  10.  How  you  see  things.  Boys,  when  you 
have  attended  movie  shows  where  they  have  animated  cartoons 
you  have  perhaps  seen  a  dotted  line  move  from  the  eye  of  the 
hero  (or  villain)  to  the  object  he  is  looking  at.  You  might  think 
from  this  that  you  see 
an  object  by  means  of 
light  which  goes  from 
your  eye  to  it.  This 
is  not  the  case,  how- 
ever, as  you  will  now 
prove. 

Take  an  unlighted 
candle  into  an  abso- 
lutely dark  room  and         Fig.  20.    You  see  the  book  by  reflected  light 

K  — 1 


GILBERT  LIGHT  EXPERIMENTS  17 


look  around.  Can  you 
see  anything?  You  can- 
not because  all  the  ob- 
jects in  the  room  are 
non  -  luminous,  includ- 
ing yourself  and  your 
eyes.  This  proves  that 
you  do  not  see  things 
by  means  of  light  which 
goes  from  your  eye  to 

the  thing  you  are  look-  Fig-  21-    LiSnt  travels  in  a  straight  line  to  your  eye 

.  ing  at.  Now  light  the  candle.  The  flame  is  a  luminous  body  and 
you  see  it  by  means  of  light  which  goes  from  it  to  your  eye. 

Can  you  now  see  the  non-luminous  bodies  ?  You  can  because 
light  travels  from  the  candle  flame  to  these  objects  and  from  them 
to  your  eye  (Fig.  20). 

You  have  proved  here  that  you  see  any  object  by  means  of 
light  which  travels  from  it  to  your  eye  and  not  the  reverse. 

Experiment  No.  11.  To  show  that  light,  when  not  reflected  or 
refracted,  travels  in  a  straight  line  from  the  object  to  your  eye. 

Note.  The  thing  you  are 
looking  at  directly  or  indi- 
rectly is  called  the  object. 

Close  one  eye,  look  at 
the  flame  of  a  candle,  and 
then  move  a  book  slowly  be- 
tween the  flame  and  your 
eye  (Fig.  21).  Do  you  find 
that  you  cannot  see  the 
flame  when  the  book  has 
crossed  the  straight  line  be- 
tween the  flame  and  your 
Fig.  22.  Light  travels  in  a  straight  line  eye  ?  This  proves  that  the 
K  — 2 


18 


GILBERT  BOY  ENGINEERING 


Fig. 


23.    You  see  an  inverted  image  of  the 
candle 


light  from  the  flame 
travels  in  a  straight  line 
to  your  eye. 

Close  one  eye  and  look 
at  any  part  of  some  other 
object,  then  again  move 
the  book  across  the 
straight  line  from  the 
part  to  your  eye.  Do 
you  find  again  that  you 
cannot  see  the  part  when 
the  book  has  crossed 
the  straight  line  between  the  part  and  your  eye? 

Cut  three  pieces  of  cardboard  about  5"X3",  punch  a  small  hole 
in  each  at  the  same  height,  stand  them  upright  on  the  table,  place 
a  candle  flame  in  front  of  an  end  hole,  and  look  at  the  flame 
through  the  three  holes  (Fig.  22).  Shift  the  cardboards  one  at 
a  time.  Do  you  find  that  you  can  see  the  flame  only  when  the 
holes  are  in  a  straight  line? 

Note.  You  will  show  later  that 
light  is  bent  out  of  the  straight  line 
when  it  is  reflected  from  a  mirror  and 
when  it  is  refracted  in  passing  from 
air  to  water,  air  to  glass,  and  so  on. 

You  have  shown  here  that  light, 
which  is  not  reflected  or  refracted, 
travels  in  a  straight  line  from  the 
object  to  your  eye. 

Experiment  No.  12.  Picture  of  a 
pandle  flame.  Punch  a  nail  hole  in 
card  C  and  arrange  as  shown  in  Fig. 
23.  Do  you  see  an  inverted  image  of 
the  flame  and  is  it  larger  the  farther    Fig.  24.  Flash-light  telegraphing 


GILBERT  LIGHT  EXPERIMENTS  19 


INTERNATIONAL  CODE  AMERICAN  MORSE  CODE 


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fSBS  fib  A 
BF  W 

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JL. 

A 

Jr 

A  A  A 

w  w  bbb  w 

m  mm  • 

091  810  0 

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W  WeES  QSB  BBA 

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wBm  W  BBBI 

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Jr* 

a  mmm  mmm  jm% 

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mmm  mmm  a  BBA 
BBBI  BBBI  B9  BBA 

a  a  m  0 

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-»-> 

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A     A  A 

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BBBI  :*-  IB 

AAA  A 

& 

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1 

A  BBA  AAJ  BBA  ARB 
Bf  BBA  BNB  BAB  BBA 

0  00  AJJ  A 

A  A  BAJ  BBBI  wsm. 
W  W  BBA  BBA  BBA 

a 

AAA  BB  BBA 

w  w  w  ■■■■  wmm 

•  O  0  OB  0 

-t 

0  •  0  0  BB 

6 

0  0  0  0  0 

o 

■00  0  0  0 

00  00  0  •  0 

■  ■n  #  • 

Ml  0  0  0  0 

mm  0  0  n 

20 


GILBERT  BOY  ENGINEERING 


D  is  from  C?  Make  the  hole  larger. 
Is  the  image  brighter  but  more 
blurred?  Can  you  explain  these 
facts  as  in  Experiments  5  and  6  and 
as  illustrated  in  Figs.  11  and  23? 

Experiment  No.  13.  Flash-light 
telegraphing.  You  can  telegraph 
to  your  friends  at  night  by  means  of 
flash-light  signals  (Fig.  24)  and  the 
Morse  code.  Use  a  short  flash  for 
a  dot  and  a  long  flash  for  a  dash. 
For  the  Morse  code  see  page  19. 
Experiment  No.  14.  Telegraph- 
Fig.  25.  Light  telegraphing      ing  with  a  candle  or  lamp.  Arrange 

the  apparatus  as  shown  in  Fig.  25 
and  have  a  friend  make  a  similar  arrangement  in  a  window  facing 
you.  You  can  then  telegraph  by  means  of  the  Morse  code.  Un- 
cover the  light  for  a  short  time  to  produce  a  dot  and  for  a  longer 
time  for  a  dash. 


INTENSITY  OF  LIGHT 

If  you  hold  a  book  1  foot  from  a  lighted  candle,  it  receives  a 
certain  amount  of  light ;  if  you  hold  it  2  feet  from  the  candle,  it 
receives  only  one-fourth  as  much  light ;  if  you  hold  it  3  feet  from 
the  candle,  it  receives  only  one-ninth  as  much  light,  and  so  on. 
That  is,  the  intensity 
of  the  light  on  any  ob- 
ject varies  inversely  as 
the  square  of  the  dis- 
tance between  the  ob- 
ject and  the  source  of 
light. 

In  Fig.  26  the  light 


Fig.  26.  The  light  which  covers  one  square  at  one 
foot  covers  four  and  nine  at  two  and  three  feet, 
and  is  then  only  one-fourth  and  one-ninth  as 
intense 


GILBERT  LIGHT  EXPERIMENTS  21 


which  would  cover  1  square 
at  1  foot  would  cover  4 
and  9  equal  squares  at  2 
and  3  feet  and  would  there- 
fore be  one-fourth  and  one- 
ninth  as  intense. 

Experiment  No.  15.  To 
prove  the  law  of  intensity. 
Screen  A,  Fig.  27,  has  a 
hole  just  1  inch  square  and 
screen  B  a  square  3  inches 
on  each  side,  divided  into  9 


Fig. 


28.    The  spot  is  dark  by  re- 
fleeted  light 


Fig.  27.  The  light  is  one-ninth  as  strong  on 
B  as  on  A  when  B  is  3  feet  from  the  candle 

square  inches.  Place  A  1  foot  from 
the  candle  and  B  2  feet.  Does  the 
light  which  passes  through  1 
square  inch  in  A  cover  4  square 
inches  on  B?  Is  it,  therefore,  only 
one-fourth  as  intense  on  B  as  it  is 
on  A?  Place  B  3  feet  from  the 
candle.  Does  the  light  now  cover 
9  square  inches?  Is  it,  therefore, 
only  one-ninth  as  intense?  This 
proves  that  the  intensity  of  light 
varies  inversely  as  the  square  of 
the  distance. 

Experiment  No.  16.  A  greased 
spot.  Rub  a  small  piece  of  butter 
on  the  center  of  a  piece  of  paper 
and  melt  it. 

Look  at  the  spot  by  reflected 
light  (Fig.  28).  Is  the  spot  dark? 
Look  at  it  by  transmitted  light 


22 


GILBERT  BOY  ENGINEERING 


(Fig.29).  Is  it  bright?  It  is  darker 
than  the  paper  in  the  first  case  and 
brighter  in  the  second,  because  more 
light  goes  through  the  greased  spot 
than  through  the  paper. 

Experiment  No.  17.  Four  can- 
dles against  one.  Put  four  candles 
2  feet  from  the  greased-spot  screen 
and  one  candle  1  foot  on  the  other 
side  (Fig.  30).  Trim  the  wicks 
until  the  flames  are  of  equal  size. 

Is  the  greased  spot  as  bright 
as  the  paper?  It  is,  because  the 
light  which  goes  through  from  one 
side  is  exactly  equal  to  that  which 
goes  through  from  the  other,  that 
is,  the  one  candle  throws  as  much 
light  on  the  screen  as  do  the  four 
candles.    This  is  explained  by  the 


Fig.  29.  The  spot  is  bright  by  trans- 
mitted light 


law  of  intensity  men- 
tioned above.  Each  of 
the  four  candles  is  2  feet 
from  the  screen  and 
therefore  throws  just 
one-fourth  as  much  light 
on  the  screen  as  one 
candle  does  at  1  foot. 

Experiment  No.  18. 
Candle  power  of  a 
lamp.  The  candle 
power  of  a  lamp  is  the 


Fig.  30. 


One  candle  at  one  foot  is  equal  to  four  at 
two  feet 


GILBERT  LIGHT  EXPERIMENTS  23 


number  of  times  greater 
or  less  its  light  is  than  that 
given  by  a  standard  candle. 

Put  the  greased  -  spot 
screen  justlfoot  fromacan- 
dle  (Fig.  31)  and  move  the 
lamp  until  the  greased  spot 
is  as  bright  as  the  paper. 
Measure  the  distance  from 
the  lamp  to  the  screen.  If 
this  is  2  feet,  the  lamp  is  Fig-  81-  mn^s  the  candle  power  of  the  lamp 
4-candle  power;  if  3  feet,  9-candle  power;  4  feet,  16-candle  power; 
and  so  on.   This  follows  from  the  law  of  intensity  of  light. 

The  instruments  used  to  measure  the  candle  power  of  lamps 
are  called  photometers,  and  the  one  you  have  here  illustrated  is 
named  after  its  inventor,  Bunsen's  photometer  (Fig.  32).  A  is  the 
greased-spot  screen,  B  the  standard  candle,  and  C  the  light  tested. 

Experiment  No.  19.  Four  against  one.  Place  a  candle  1  foot 
from  a  screen  and  four  equal  candles  just  2  feet  from  the  screen, 
and  place  an  object  in  front  of  the  screen  as  in  Fig.  33.  Are  the 
shadows  of  equal  darkness? 

If  the  candle  flames  are  all  of  the  same  size,  the  shadows  are 
equally  dark  because  each  light  illuminates  the  shadow  produced 
by  the  other  and  because  the  candle  at  1  foot  sends  as  much  light 

to  the  screen  as 
the  four  candles 
do  at  2  feet  from 
the  screen.  This 
is  explained  by  the 
law  of  intensity. 

E  x  p  e  r  i  ment 
No.  20.  Shadow 

Fig.  32,    Bunsen's  photometer  photometer.  Put  a 


24 


GILBERT  BOY  ENGINEERING 


candle  1  foot  from  a  screen 
(see  Fig.  34,  illustrating  shad- 
ow photometer),  stand  a  pen- 
cil in  front  of  the  screen,  and 
move  the  lamp  back  and  forth 
until  the  two  shadows  are 
equally  intense.  Measure  the 
distance  from  the  screen  to 
the  lamp.  If  this  is  2,  3,  4,  or 
A}/2  feet,  the  candle  power  of 
the  lamp  is  4,  9,  16,  or  20%, 
and  so  on. 

Each  light  illuminates  the 
shadow  cast  by  the  other  and 
therefore  when  the  shadows  are  equal  the  lights  are  equal,  and  the 
candle  power  is  calculated  as  above  from  the  law  of  intensity  of 
light.    This  illustrates  the  shadow  photometer. 


Fig.  33.   One  candle  equal  to  four 
From  Millihan  and  Gale's  First  Course  in 
Physics,  Ginn  &  Co. 


SHADOWS 


Experiment  No.  21. 

Enlarging  shadow. 
Move  a  pencil  from  the 
screen  toward  the 
candle  (Fig.  35).  Does 
the  shadow  increase  in 
size?  It  does,  because 
light  goes  in  all  direc- 
tions in  straight  lines 
from  the  flame,  and  the 
pencil  intercepts  more 
light  the  nearer  it  is  to 
the  flame. 


Fig.  34.    Illustrating  the  shadow  photometer 


GILBERT  LIGHT  EXPERIMENTS  25 


SHADOW  ENTER- 
TAINMENTS 

Boys,  you  can  have 
the  greatest  kind  of  fun 
bygiving  shadow  shows 
to  your  friends,  and  the 


Fig.  35.  Shadows 

preparation  you  need  is  very  slight. 
Hang  a  sheet  over  a  folding  doorway 
as  shown  in  Fig.  36. 

Now  opposite  the  door  put  a  strong 
lamp  on  a  stool,  chair,  or  table,  accord- 
ing to  the  show  (Fig,  37).  The  audi- 
ence is  in  darkness  on  the  other  side 
of  the  screen. 

Show  1.    The  Dentist.  Dentist 


Fig.  36.    A  sheet  over  a 
doorway- 


Seated,  bell  rings,  boy  comes  in 
with  bandaged  head,  dentist 
seats  him  and  examines  tooth, 
boy  howls,  dentist  takes  very, 
very  large  pliers,  and  pulls  out 
a  very  large  cardboard  tooth 
(Fig.  38).  The  tooth,  of  course, 
was  stuck  in  boy's  coat  collar 


Fig.  37.    Showing  lamp  on  a  chair 


26  GILBERT  BOY  ENGINEERING 


Fig.  38.    The  dentist 

heart,  all  with  much  panto- 
mime, takes  large  coal  tongs, 
shoves  them  down  boy's  throat 
(apparently,  of  course),  and 


at  one  side.  Use  much  panto- 
mime all  through  the  show. 

Show  2.  The  Doctor.  Doctor 
seated  with  very  large  plug  hat 
and  very  long  beard,  bell  rings, 
boy  enters  rubbing  stomach  and 
groaning.  Doctor  seats  him,  takes 
pulse,  examines  tongue,  listens  to 


Fig.  40.    How  it  is  done 


Fig.  3U.    The  doctor 

pulls  up  a  long  snake  (Fig.  39). 
More  pantomime.  Boy  not  yet 
well,  doctor  again  shoves  tongs 
down  his  throat  and  pulls  up  an 
alligator,  and  so  on.  Much  panto- 
mime of  boy  feeling  fine. 

The  snake  and  alligator  are  cut 
out  of  stiff  paper  or  cardboard  and 
are  handed  up  by  a  third  boy  as 
shown  in  Fig.  40, 


GILBERT  LIGHT  EXPERIMENTS  27 


Show  3.    A  Surgical  Case. 

Scene  1.  A  boy  is  seated  at 
a  table  with  large  plate  of  pota- 
toes. He  swallows  them  whole, 
then  swallows  knife,  fork,  spoon, 
saltcellar,  and  so  on.  Much  pan- 
tomimeof  enjoyingagood  meal. 

Scene  2.  Doctor  seated  at 
table,  boy  rushes  in  rubbing 
stomach,  doctor  lays  him  on 
table,  takes  large  knife,  jabs  it 


Fig.  42.    Quieting  the  patient 


ing  fine,  shakes  hands  with 
doctor  and  thanks  him. 

The  knife  and  axe  are  cut 
out  of  cardboard,  the  plug  hat 
is  a  tube  of  stiff  paper  on  an  or- 
dinary hat,  the  whiskers  are  an- 
other tube  of  paper.  The  boy 
swallowing  potatoes  really 
hands  them  to  another  boy  hid- 
den beside  the  chair. 

Show  4.  A  Boxing  Match. 


Fig.  41.    A  surgical  case 


into  boy's  stomach  (Fig.  41), 
boy  raises  head  to  object,  doc- 
tor hits  him  on  head  with  hatch- 
et (Fig.  42)  and  proceeds  to  cut 
him  open,  throws  back  coat 
to  imitate  opening  stomach, 
and  takes  out  all  the  potatoes, 
knife,  fork,  spoon,  and  so  on. 
Doctor  sews  boy  up,  hits  him 
on  head  with  hatchet,  boy 
comes  to,  pantomime  of  feel- 


Fig.  43.   A  boxing  match 


28 


GILBERT  BOY  ENGINEERING 


Fig.  44.    Living  shadows 


Put  one  boy  near  the 
screen  and  another 
nearer  the  light.  The 
first  is  natural  size, 
the  second  is  enor- 
mous (Fig.  43).  If  they 
now  pretend  to  fight 
it  is  very,  very  funny 
from  the  audience.  In 
one  of  the  fights,  have 
the  lamp  on  the  stool, 
let  the  little  fellow 
beat  the  big  fellow, 
and  if  the  big  fellow  finally  runs  away  and  Steps  over  the  lamp 
to  the  chair  it  looks  as  though  he  had  jumped  into  the  ceiling. 
Little  fellow  then  struts  around  as  winner. 

Show  5.  Living  Shadows.  Cover  a  mirror  with  two  pieces 
of  paper,  out  of  each  of  which  you  have  cut  identical  eyes, 
nose,  and  mouth  with  teeth,  as  shown  in  Fig.  44.  Paste 
the  under  paper  against  the  mirror,  but  paste  the  outer  paper 
only  at  the  top.  Arrange  the  light  and  boy  as  shown  and  sway 
the  outer  paper  back  and  forth.  Do  you  see  goggling  eyes  and 
snapping  mouth? 

Now  have  the  boy,  whose  shadow  is  shown,  make  a  speech 
with  proper  gestures,  while  you  sway  the  paper.  The  effect 
will  be  extremely  amusing  to  the  spectators. 

Show  6.  Living  Shadow  Dialogue.  Arrange  two  mirrors 
as  above  and  place  one  on  each  side  of  the  screen.  Have  the 
two  shadows  carry  on  a  dialogue  while  two  other  boys  sway 
the  papers. 

You  will  have  plenty  of  fun  inventing  shows  of  your  own, 
and  with  a  few  beards,  mustaches,  and  false  noses  made  of  paper 
or  of  other  material  you  can  have  very,  very  funny  times. 


GILBERT  LIGHT  EXPERIMENTS  29 


Fig.  45.  The  angle 
of  reflection  R  ia 
equal  to  the  angle 
of  incidence  I 


REFLECTION  OF  LIGHT 
The  Law  of  Reflection:  Angle  of  reflection  equals  angle  of 
incidence.  If  a  beam  of  sunlight  is  allowed  to  fall  on  a  mirror 
and  the  beam  before  and  after  reflection  is  made  visible  by  dust 
in  the  air,  it  is  found  that  the  beams  make  equal  angles  with  a 
ruler  held  perpendicular  to  the  mirror,  and 
that  they  are  in  the  same  plane.  The  beam 
I,  Fig.  45,  which  strikes  the  mirror  is  called 
the  incident  beam,  and  the  beam  R  which  is 
reflected  is  called  the  reflected  beam.  The 
angle  i  which  the  incident  beam  makes  with 
the  perpendicular  PN  is  called  the  angle  of 

incidence,  and  the  angle  r  which  the  reflected  beam  makes  with 
the  perpendicular  is  called  the  angle  of  reflection.  This  experi- 
ment illustrates  the  Law  of  Reflection,  which  is :  The  angle  of 
reflection  equals  the  angle  of  incidence  and  the  reflected  and 
incident  beams  are  in  the  same  plane. 

FUN  WITH  SUNLIGHT 
Experiment  No.  22.    To  prove  the  law  of  reflection.  Allow 
a  small  beam  of  sunlight  to  pass  through  the  slit  of  your  shutter 

and  fall  on  a  mirror 
placed  on  the  floor 
or  table  of  your 
darkened  room. 
Make  sufficient  dust 
to  show  the  sunlight. 
Is  the  sunlight  re- 
flected and  does  it 
make  a  bright  spot 
on  the  ceiling  or  op- 
posite wall? 

Fig.  46.    The  beams  make  equal  angles  with  ,  T       ,    ,  ,  , 

your  ruler  JNow  hold  a  ruler 


30 


GILBERT  BOY  ENGINEERING 


Fig.  47.    The  beams  are  in  the  same  plane 


perpendicular  to  the 
mirror  opposite  the 
spot  where  the  light 
strikes  the  mirror  (Fig. 
46).  Do  you  find  that 
the  angle  between  the 
reflected  beam  and  the 
ruler  is  equal  to  the 
angle  between  the  in- 
cident beam  and  the 
ruler? 

Hold  a  sheet  of 
cardboard  edge-wise  to  the  incident  and  reflected  beams  in  such  a 
position  that  the  incident  beam  is  split  in  two  (Fig.  47).  Is  the 
reflected  beam  also  split  in  two?  That  is,  are  the  reflected  and 
incident  beams  in 
the  same  plane? 

You  have  here 
proved  the  law  of 
reflection. 

Experiment  No.  23. 
Irregular  reflection. 
Let  the  beam  fall  on 
a  piece  of  white  un- 
glazed  paper  (Fig. 
48).  Is  there  no  re- 
flected beam ;  is  the 

light  reflected  in  all  directions  and  does  it  make 
everything  around  it  brighter?  The  light  is 
reflected  in  all  directions  because  the  surface 

riS^?flectiongUlar       is  rOUgh   (See  Fi§-'  49)' 

fngh  8chooinphy8-  You  see  a11  non^m'mous  objects  by  means 
%?  &  VSuS    of  %ht  which  they  reflect  irregularly. 


Fig.  48.  There  is  no  reflected  beam  from  rough  paper 


GILBERT  LIGHT  EXPERIMENTS  31 


Experiment  No.  24.  Twice  the 
angle.  Hold  the  mirror  perpen- 
dicular to  the  beam.  Is  the  beam 
reflected  back  to  the  slit?  Now 
turn  the  mirror  to  an  angle  of  45° 
to  the  beam.  Is  the  reflected  beam 
turned  through  an  angle  of  90°? 
That  is,  is  the  reflected  beam 
turned  through  twice  the  angle 
the  mirror  turns?  Try  other  an- 
gles. The  reflected  beam  turns 
through  twice  the  angle  because 
the  angles  of  incidence  and  reflec- 
tion are  equal  and  each  is  equal 
to  the  angle  through  which  the 
mirror  turns,  therefore,  together 
they  are  equal  to  twice  this  angle. 


Fig.  50.  HeliograpMng 


FUN  BY  DAY  WITH  ONE  MIRROR 
Experiment  No.  25.  The  heliograph.    Reflect  sunlight  from 
your  window  (Fig.  50)  to  a  distant  building,  and  have  your  friend 
reflect  sunlight  from  near  this  building  to  your  window. 

Now  send  a  message  to  your  friend  by  the  Morse  code.  Un- 
cover your  mirror  for  a  short  time  for  a  dot  and  for  a  longer 
time  for  a  dash.  He  reads  the  message  on  the  wall  of  the  build- 
ing. He  replies  and  you  read  the  message  on  the  inside  wall  of 
your  room.  This  is  the  principle  of  the  heliograph  used  for 
military  signaling. 

Experiment  No.  26.  Height  of  any  point  on  a  building.  Drive 
one  end  of  a  straight  stick  into  the  ground  and  make  the  stick 
exactly  vertical.  Place  the  mirror  B,  Fig.  51,  beside  it  flat  on  the 
ground  and  adjust  until  the  stick  and  its  image  are  in  a  straight 
line ;  the  mirror  is  then  exactly  horizontal. 


32 


GILBERT  BOY  ENGINEERING 


J 


Fig.  51.   You  find  the  height  of  the 
window  by  reflection 


Now  if  you  want  to  find  the 
height  of  the  topmost  window,  for 
example,  stand  back  until  you  can 
just  see  the  top  of  the  window, 
then  measure :  your  distance  BC 
from  the  mirror ;  the  distance  BE 
from  the  mirror  to  the  building; 
and  your  height  CA  from  sole  to 
eye. 

The  triangle  ABC  which  you 
make  with  the  mirror  is  similar  to 
the  triangle  DBE  which  the  top  of 
the  window  makes  (Fig.  51),  that 
is,  they  have  equal  angles,  there- 
fore, 

DE  AC 


EB 


CB 


Examples.  If  you  are  12  feet 
from  the  mirror,  your  height  to 
your  eye  is  5  feet,  and  the  mirror 


is  120  feet  from  the  building  at  the  ground  level,  then 
Height  of  window  5 

12 


Height  of  window 


120 
5  X  120 

12 


50  feet 


FUN  BY  DAY  OR  NIGHT  WITH  ONE  MIRROR 

Experiment  No.  27.  An  object  and  its  image.  Look  at  yourself 
in  a  vertical  mirror  and  move  back  and  forth.  Does  your  image 
always  appear  to  be  as  far  behind  the  mirror  as  you  are  in  front  ? 


GILBERT  LIGHT  EXPERIMENTS  33 


Fig.  52. 


The  candles  are  at  equal  distances  from  your 
mirror 


Arrange  the 
window  glass  ver- 
tically, place  a  can- 
dle in  front  and  an- 
other behind  (Fig. 

52)  ,  and  make  the 
rear  candle  coin- 
cide with  the  image 
of  the  front  candle. 
Measure  the  dis- 
tance from  each 
candle  to  the  mir- 
ror. Are  they  ex- 
actly equal? 

Draw  a  line  on  a  piece  of  paper  and  call  it  the  mirror  line  (Fig. 

53)  .  Draw  three  lines  across  it,  and  perpendicular  to  it,  at  2-inch 
intervals.  Place  the  window  glass  vertically  on  the  mirror  line,  place 

the  front  candle  on  each  perpendicular 
in  turn.  Is  its  image  candle  always  at 
the  same  distance  from  the  mirror? 

You  have  proved  here  that  an 
image  is  always  the  same  distance 
behind  the  mirror  that  the  object 
is  in  front;  and  that  it  is  on 
pendicular  drawn  from  the 
across  the  mirror  line. 
Experiment  No.  28.  Slanting  object. 
Place  a  pencil  in  front  of  the  vertical 
window  glass  (Fig.  54),  and  slanting 
toward  the  glass.  Make  a  second  pencil 
of  equal  length  coincide  with  the  image. 
Does  the  image  also  slant  toward  the 
mirror?   It  does,  because  each  part  of 


Fig.  53.    The  candles  are  on 
the  same  perpendicular 


a  per- 
object 


Fig. 


54.  Both  pencils  slant 
toward  your  mirror 


K  —  3 


34  GILBERT  BOY  ENGINEERING 


the  image  is  as  far  be- 
hind as  the  corre- 
sponding part  of  the 
object  is  in  front,  and 
both  are  on  a  line  per- 
pendicular to  the  mir- 
ror line. 

Experiment  No.  29. 
To  copy  a  drawing. 
Arrange  drawing,  ver- 
tical   window  glass, 

Fig.  55.   You  copy  a  drawing  easily  book,  paper,  and  light, 

as  shown  in  Fig.  55.  Do  you  find  it  easy  to  copy  the  drawing? 
Why  is  the  drawing  reversed? 

WHY  THE  IMAGE  IS  AS  FAR  BEHIND  THE  MIRROR 
AS  THE  OBJECT  IS  IN  FRONT 

We  will  explain  this  first  by  means  of  rays  and  then  by  means 
of  waves,  but  you  must  remember  that  what  you  actually  receive 
in  your  eyes  is  light  waves  and  not  rays.  The  rays  are  only  im- 
aginary lines  which  show  the  direction  the  waves  are  moving. 

In  Fig.  56  the  eye  sees  image  B,  but  the  light,  of  course,  goes 
from  A  to  the  mirror  and  is  reflected  to  the  eye.  The  angles  r 
and  i  are  equal  by  the  law  of  reflection ;  also,  since  CD  and  AB 
are  parallel,  angle  A  is  equal 
to  i  and  angle  B  is  equal  to  r. 
Therefore  angles  A  and  B  are 
equal ;  also  the  angles  at  F  are 
equal,  since  they  are  right  an- 
gles. The  triangles  CFA  and 
CFB  then  have  two  angles 
equal  and  a  side  CF  in  com- 


2* 


mon,  therefore  they  are  equal  Fig.  5e.  The  ray  appears  to  come  from  b 


GILBERT  LIGHT  EXPERIMENTS  35 


Fig.  57.  The  waves  appear  to  come  from  B 


and  FB  is  equal  to  AF.  This 
shows  that  the  image  is  as  far 
behind  the  mirror  as  the  object 
is  in  front  because  by  the  law 
of  reflection  r  is  equal  to  i. 

We  will  now  explain  by 
means  of  light  waves  why  the 
image  and  objects  are  at  equal 
distances  from  the  mirror,  as 
follows : 

The  waves  of  light  from  the 
candle  A  strike  the  mirror  as  in 
1,  Fig.  57,  and  are  reflected  as 

in  2.  The  curvature,  of  the  waves  is  exactly  reversed  by  reflec- 
tion. The  eye  estimates  the  distance  of  an  object  partly  by  the 
curvature  of  the  waves  which  enter  it,  and  the  image  appears  at 
B  as  far  behind  the  mirror  as  the  object  at  A  is  in  front,  because 
the  reflected  waves  which  enter  the  eye  have  exactly  the  curva- 
ture they  would  have  if  the  mirror  were  absent  and  the  object 
were  at  B. 

We  see,  then,  that  the  image  of  A  is  at  an  equal  distance  B 
because  the  waves  from  A  are  reversed  by  the  mirror  but  un- 
altered in  any  other 
way. 

Experiment  No.  30. 

To  illustrate  reflected 
waves.  Fill  with  wa- 
ter a  cake  tin  with  flat 
sides  (Fig.  58),  place 
it  near  a  good  light, 
dip  a  pencil  in,  two 
inches  from  one  side. 
Fig.  58.   You  see  circular  waves  reflected  Is  the  reflected  wave 


.  . 

4k 

■f  : 

4 

■ppp 

':.   :  ;.-..":-V,/ 

%       ,     ~~  'J 

36 


GILBERT  BOY  ENGINEERING 


Fig.  59. 


Your  right  hand  is  an  image  of  your  left 
and  vice  versa 


curved  in  the  opposite 
direction  to  the  orig- 
inal wave?  Is  it 
larger  and  is  it  the 
size  it  would  be  if  it 
came  from  a  point  at 
an  equal  distance  out- 
side the  pan?  This  is 
what  happens  when  a 
light  wave  strikes  a 
mirror. 

Experiment  No.  31. 

Why  your  image  is  reversed.  Hold  your  two  hands  in  front  of 
you  (Fig.  59).    Is  one  a  reversed  image  of  the  other? 

Hold  your  left  hand  in  front  of  a  mirror  (Fig.  60).  Does  it 
appear  to  be  your  right  hand?  It  does,  because  each  point  of 
the  image  is  the  same  distance  behind  the  mirror  that  the  corre- 
sponding part  of  the  left  hand  is  in  front. 

Experiment  No.  32.  Reversed  words.  Print  such  words  as 
STAR,  STUN,  and  TOP  on  paper  and  look  at  their  images 
(Fig.  61).    Are  they  reversed?  Why? 

Experiment  No.  33. 
Reading  a  blotting  pa- 
per. Write  a  sentence 
in  ink,  blot  it  on  fresh 
blotting  paper.  Try 
to  read  it.  It  is  hard. 
Read  it  in  the  mirror 
(Fig.  62).  Is  it  easy? 
Why? 

Experiment  No.  34. 

To  see  behind  you. 
TT         ^  .  .        Fig.  60.    Your  left  hand  appears  to  be  the  image 

Hold  the  mirror  in  of  the  right 


GILBERT  LIGHT  EXPERIMENTS  37 


front  of  you  as  in  Fig.  63. 
Can  you  see  behind  you 
with  ease?  Why? 

Experiment  No.  35. 
To  see  around  a  corner. 
Hold  a  mirror  as  in 
Fig.  64.  Can  you  see 
around  a  corner  with 
ease?  Why? 

EXPERIMENTAL 

Fig.  61.    You  see  a  different  word 

MAGIC 

Experiment  No.  36.  A  candle  burning  in  a  glass  of  water. 
Place  candle  in  front  of  window  glass  and  glass  of  water  behind  it, 
as  in  Fig.  65.    Does  the  candle  appear  to  burn  in  the  water? 

Experiment  No.  37.  Phantom  candle  in  boy's  head.  Arrange 
apparatus  as  in  Fig.  66.  Does  the  candle  appear  to  burn  in  the 
boy's  head? 

Experiment  No.  38.  Phantom  flame.  Arrange  the  apparatus 
as  in  Fig.  67  and  hold  your  hand  behind  in  the  image  of  the  flame. 
Can  you  do  this  quite  safely? 

Experiment   No.  39.     To  make  magical  transformations. 


Arrange  the  appa- 
ratus as  in  Figs.  68 
and  69.  Place  the 
lighted  candle  in  po- 
sition 1  and  adjust 
block  A  until  it  coin- 
cides with  the  image 
of  block  B. 

Now  to  prepare  a 
transformation,  fold 


Fig.  62.   You  read  a  blotting  paper  easily  a  sheet  of  paper  once 


38 


GILBERT  BOY  ENGINEERING 


Fig.  63.    You  see  behind  your  back 


over  at  the  middle, 
and  on  the  top  half 
make  a  drawing  of  a 
man.  Make  it  with  a 
lead  pencil  and  bear 
down  so  as  to  crease 
the  lower  half.  Now 
draw  on  the  lower 
half  inside  the  creases 
the  skeleton  of  a  man. 
Tear  the  halves  apart, 
attach  the  man  to  B 
and  the  skeleton  to  A, 
and  adjust  until  the 
man  seems  to  turn  into  a  skeleton  when  you  move  the  candle 
from  position  1  to  position  2. 

Now  darken  the  room  and  ask  a  friend  to  look  into  the  glass, 
as  in  Fig.  69,  while  you  move  the  candle  from  1  to  2.  He  will  be 
much  mystified. 

Use  photos  of  the  same  size,  one  of  a  boy  and  the  other  of  a 
girl,    and    make  the 
transformation.  Make 
many     other  trans- 
formations. 

You  may  have  seen 
similar  magical  trans- 
formations  at 
the  theater  or  in  side 
shows. 

Experiment  No.  40. 
A  magic  box.  Make 
out  of  stiff  cardboard 

a  square  box  24  inches  Fig.  64.   You  see  around  a  corner 


GILBERT  LIGHT  EXPERIMENTS  39 


Fig.  65.   You  see  the  candle  burning  in  water 


long  and  3^4  inches 
wide  and  high.  Cut 
the  box  into  two  equal 
parts  at  an  angle  of 
45°  (see  1,  Fig.  70), 
turn  the  halves  as  in 
2,  cut  a  hole  in  one 
half,  its  center  being 
2  inches  from  the  end. 
Cut  two  trapdoors 
and  attach  to  each  a 
loop  of  cord  for  open- 
ing it.  Now  insert  the 
window  glass  at  the  division,  see  3. 

Now  to  have  fun  with  your  friends,  hide  the  box  behind  a 
large  sheet  of  paper  or  cardboard  and  have  your  friends  look 
through  a  hole  in  the  paper  or  cardboard  into  the  hole  in 
the  box. 

Have  the  box  in  a  good  light  and  have  different  objects  be- 
neath the  trapdoors.  If  you  now  open  one  trapdoor,  the  object 
beneath  is  seen  by  your  friends.  If  now  you  close  the  first  door 
and  open  the  second,  the  object  will  appear  to  be  transformed 

into  the  other  ob- 
ject. 

Use  an  empty 
glass  and  a  glass  half 
full  of  milk  and  you 
can  make  the  tumbler 
empty  and  fill  at 
will. 

You  can  make 
many  other  very  fun- 
ny transformations. 


Fig.  66.  You  see  the  candle  burning  in  the  boy's  head 


40 


GILBERT  BOY  ENGINEERING 


Fig. 


to 


FUN  AT  NIGHT 

Hypnotism 

Boys,  you  can  put  on  imitation 
hypnotism  shows  which  will  mystify 
and  amuse  your  friends,  as  follows : 
Get  a  piece  of  window  glass 
2  feet  by  3  feet,  or  larger.  Put  it  in 
a  box  (Figs.  71  and  72)  at  an  angle 
of  45°  to  the  length  of  the  box  and 
exactly  vertical.  Paint  the  inside  of 
the  box  black.  The  box  is  open  to- 
ward the  audience,  also  at  the  op- 
posite end,  Bl,  B2,  B3,  and  at  the 
side,  Al,  A2,  A3.  A  strong  light  1 
is  placed  at  the  side  and  throws  light 
on  a  person  sitting  opposite  Al,  A2, 
or  A3,  but  not  on  the  glass.  A 
weaker  light  2  is  placed  so  that 
it  will  throw  light  on  a  person 
seated  opposite  Bl,  B2,  or  B3,  t 
but  not  on  the  glass.  When 
light  1  is  up,  a  boy  seated  oppo- 
site Al,  A2,  or  A3  will  be  seen 
by  the  audience  by  reflected 
light  as  seated  at  Bl,  B2,  or  B3. 
When  light  2  is  up.  a  boy  seated 
at  Bl,  B2,  or  B3  will  be  seen  at 
Bl,  B2,  or  B3  by  direct  light. 
The,  direct  image  is  brighter 
than  the  reflected,  and  to  cut  it 
down  tack  two  or  three  layers 
of  green   or  black  mosquito 


67.    The  candle  appears 
burn  in  your  hand 


SKELETON 


WINDOW  GLASS 

Fig.  68.  Transformations 


GILBERT  LIGHT  EXPERIMENTS  41 


netting  over  the  B 
opening*.  Use  black 
backgrounds  behind  A 
and  B;  this  is  impor- 
tant. 

You  should  always 
try  out  your  appa- 
ratus just  before  you 
use  it.  Look  at  the 
window  glass  from  Fig  69  How  you  arrange  glass>  book>  pictures, 
the  audience  side,  have  and  candles 

both  lamps  lighted,  and  the  boy  A  will  appear  to  be  seated  beside 
boy  B.  If  A  appears  too  low,  tilt  the  glass  toward  A ;  if  he  ap- 
pears too  high,  tilt  the  glass  toward  B.  If  the  glass  is  exactly 
vertical,  A  and  B  will  be  at  exactly  the  same  height. 


Fig.  70.    How  to  make  a  magic  box 
From  Magical  Experiments,  published  by  David  McKay  Co.,  Philadelphia 


42 


GILBERT  BOY  ENGINEERING 


Now  have  an  assistant  turn  down  lamp  1  and  A  will  dis- 
appear, turn  up  lamp  1  and  A  appears  again.  Also  turn  light 
2  down  and  up  and  B  disappears  and  reappears. 

These  shows  need  five  boys.  One  is  the  hypnotist,  who 
stands  out  in  front  and  gives  the  patter  talk  to  the  audience. 
He  can  wear  a  dress  suit,  mustache,  tall  hat,  and  so  on,  if  he 
desires.  Two  boys  are  needed  as  the  hypnotism  subjects,  who 
occupy  chairs  A  and  B,  as  required.  The  remaining  two  boys 
are  needed  to  operate  the  lamps. 

You  will  invent  all  kinds  of  shows  for  yourselves  and  make 
up  your  own  patter  talk,  but  we  suggest  a  few  that  will  make 
plenty  of  fun. 

Show  1.  To  hypnotize  a  boy  at  a  distance  of  ten  feet  and  to 
make  him  disappear  and  reappear. 


THE  BOX  IS   BLACK  ON  THE  INSIDE  - 

WINDOW  I 


STRONG  UCHT 


Fig.  71.    Your  hypnotism  box 


GILBERT  LIGHT  EXPERIMENTS  43 


TABLE 


Fig.  72.    Plan  of  hypnotism  apparatus 


Patter.  "Ladies  and  Gentlemen,  I  have  much  pleasure  in 
announcing  that  I  will  to-night  give  you  an  exhibition  of  hyp- 
notic power  more  wonderful  than  anything  you  have  yet  seen. 
It  may  surprise  those  of  you  who  know  me  to  learn  that  I  have 
been  studying  hypnotism  for  years,  and  it  will  surprise  you  still 
more  to  learn  that  I  have  discovered  a  secret  source  of  hypnotic 
power  of  immensely  greater  strength  than  any  such  power  dis- 
covered up  to  this  time.  Former  hypnotists  have  hypnotized 
their  subjects  at  a  distance  of  one  or  two  feet ;  but  I  dare  not  do 
this  because  my  power  is  so  great  that  it  might  injure  the  subject. 
I  will  do  all  my  work  at  a  distance  of  ten  feet,  which  I  have  found 
to  be  a  perfectly  safe  distance.  I  do  not  care  to  explain  my  power 
any  more  than  to  say  that  I  have  in  the  cabinet  powerful  bodies 


GILBERT  BOY  ENGINEERING 


which  bathe  my  subjects  in  electro-magnetic  forces  and  help 
me  in  my  work."  (Note.  This  is  perfectly  true  because  lamps 
give  out  light  and  light  is  electro-magnetic  in  nature ;  of  course, 
do  not  tell  the  audience  this.)  "Now  without  further  talk 'I  will 
give  an  exhibition  of  my  hypnotic  power  by  hypnotizing  Charles 
at  a  distance  of  ten  feet."   (Raises  curtain.) 

Note.  Charles  is  seated  at  center  position,  B2  (see  Fig.  72). 
Light  2  is  turned  up  and  1  is  turned  down.  Chair  at  side  is  in 
position  A2. 

Hypnotist :  "Charles,  attention  !"  (Makes  passes  with  hands 
slowly  and  says,  "Sleep !  Sleep !  Sleep !"  slowly.  Charles  shivers, 
gets  rigid,  and  slowly  closes  eyes.) 

Hypnotist:  "Now,  Ladies  and  Gentlemen,  former  hypnotists 
have  made  their  subjects  do  such  simple  things  as  rise  in  the  air, 
remain  floating  in  the  air,  and  so  on.  On  another  occasion  I  will 
show  you  some  of  these  simple  things,  but  I  will  now  give  you  a 
much  more  wonderful  example  of  hypnotic  power.  I  will  make 
Charles  dissolve  into  thin  air  and  disappear  entirely." 

Hypnotist  (to  Charles)  :  "Charles,  avaunt!  Avaunt!  Avaunt!" 
(slowly  with  passes).  (Lamp  2  is  slowly  turned  down  and  at  the 
same  time  1  is  slowly  turned  up.  Charles  gradually  fades  into 
nothing,  but  the  chair  is  apparently  left.) 

Hypnotist :  "Ladies  and  Gentlemen,  Charles  has  now  joined 
the  spirit  world ;  but  for  the  sake  of  his  family  and  friends,  I  will 
call  him  back.  I  must  do  this  quickly  or  he  may  get  beyond  my 
power,  great  as  this  is."  (Looking  at  ceiling)  "Charles,  appear!" 
(Looking  at  glass)  "Appear!  Appear!"  (slowly).  (Light  1  is 
slowly  turned  down  and  2  slowly  turned  up  and  Charles  slowly 
appears.) 

Hypnotist  snaps  his  fingers.  Charles  wakes  up  and  smiles. 
Hypnotist  drops  curtain,  bows  to  the  audience,  and  goes  behind 
the  curtain  to  help  arrange  for  the  next  show. 

Show  2.   To  hypnotize  a  boy,  turn  him  into  another  boy,  and 


GILBERT  LIGHT  EXPERIMENTS  45 


then  turn  him  back  again.  (Charles  is  seated  at  position  B2, 
lamp  2  is  up ;  Henry  is  seated  at  position  A2,  lamp  1  is 
turned  down.) 

Hypnotist:  "Ladies  and  Gentlemen,  I  will  now  give  you  an 
even  greater  example  of  my  hypnotic  power.  Other  hypnotists 
make  their  subjects  believe  that  they  are  some  one  else,  but  I  will 
actually  turn  my  subject  into  another  being  and  right  before 
your  eyes.  Now  watch  carefully  and  please  do  not  talk,  because 
I  have  to  concentrate  my  will  to  make  this  transformation,  and 
if  my  attention  is  diverted  my  subject  might  be  left  half  changed, 
which  would  be  very  serious  indeed." 

(Raises  curtain,  speaks  to  Charles)  "Charles,  attention !" 
(Makes  short  passes  and  says)  "Sleep!  Sleep!  Sleep!"  (slowly). 

(To  audience)  "Now,  Ladies  and  Gentlemen,  if  you  will  keep 
perfectly  quiet  I  will  change  Charles  into  another  boy." 

(To  Charles)  "Charles,  change!  Change!  Change!"  (slowly). 

(Light  2  is  slowly  turned  down  and  1  slowly  turned  up. 
Charles  turns  into  Henry  slowly.  Henry  is  also  asleep.  Hyp- 
notist snaps  fingers  —  Henry  wakes  up  and  smiles.) 

Hypnotist :  "Now,  Ladies  and  Gentlemen,  it  would  never  do 
to  leave  these  boys  mixed  up  in  this  way  because  their  mothers 
would  never  know  which  is  which,  not  to  mention  their  best 
girls,  so  I  will  turn  them  back  again.  Now  quiet,  please.  Henry, 
attention !"  (He  mesmerizes  Henry  with  passes  and  saying, 
"Sleep !  Sleep !  Sleep !"  Henry  stiffens  and  closes  eyes.  He 
then  says,  "Change !  Change !  Change !"  and  Henry  slowly 
changes  to  Charles  as  light  2  is  turned  up  and  1  down.) 

Hypnotist  wakes  up  Charles,  drops  curtain,  bows,  and  retires 
behind  curtain  again. 

Show  3.   Transmigration  of  souls. 

Hypnotist:  "Ladies  and  Gentlemen,  my  next  exhibition  of 
my  hypnotic  power  will  deal  with  the  transmigration  of  souls. 
You  have  all  heard  of  this  strange  Hindu  theory,  namely,  that 


46 


GILBERT  BOY  ENGINEERING 


our  souls  have  passed  down  the  ages  and  have  migrated  from 
one  animal  or  man  to  another.  Now  I  have  traced  Charles's 
soul  history,  and  it  is  very  interesting.  I  have  not  the  time  to 
show  you  all  the  forms  it  has  taken,  but  I  will  show  you  the 
animals  his  soul  inhabited  one  thousand,  two  thousand,  and  three 
thousand  years  ago." 

(Raises  curtain,  mesmerizes  Charles  as  before,  then  says) 
"O  ancient  soul  form !  Come !  Come !  Come !"  (Makes  passes 
and  bows  three  times  toward  Charles.) 

(Charles  slowly  changes  to  a  dog.  Charles  is  at  B2,  dog 
is  on  a  box  at  A2.  At  first  light  2  is  up  and  light  1  is  down. 
The  dog  appears  as  2  is  lowered  and  1  is  turned  up.) 

Hypnotist:  "Charles  is  a  very  nice  boy,  and  you  see  that 
one  thousand  years  ago  his  soul  inhabited  the  body  of  a  very 
nice  dog.  I  will  now  show  you  the  body  his  soul  inhabited  two 
thousand  years  ago." 

(Turns  toward  dog,  makes  passes,  bows  three  times,  and  says) 
"O  more  ancient  soul  form!  Come!  Come!  Come!" 

(While  hypnotist  is  talking  Charles  has  left  his  seat  and  a 
cat  on  a  box  is  put  in  his  place.  The  dog  now  changes  to  a  cat 
as  light  2  is  turned  up  and  light  1  down.) 

Hypnotist :  "You  see  that  Charles's  soul  two  thousand  years 
ago  occupied  the  body  of  a  very  beautiful  cat.  I  will  now  show 
you  the  body  his  soul  occupied  three  thousand  years  ago." 

(Makes  passes  toward  cat,  bows  three  times,  and  says)  "O 
most  ancient  soul  form  !  Come !  Come !  Come !" 

(The  dog  has  been  replaced  by  a  bird  in  a  cage  and  the  cat 
changes  to  a  bird  as  light  1  is  turned  up  and  light  2  down.) 

Hypnotist:  "Ladies  and  Gentlemen,  Charles's  soul  occupied 
the  body  of  a  beautiful  bird  three  thousand  years  ago.  I  will 
now  turn  the  bird  back  to  Charles.  Otherwise  the  cat  might  eat 
Charles  up,  and  I  am  afraid  Charles's  mother  would  not  forgive 
the  cat  or  me." 


GILBERT  LIGHT  EXPERIMENTS  47 


(Makes  passes  at  bird  and  says)  "Charles,  appear!  Appear! 
Appear  Vs 

(Charles  has  taken  his  place  again  at  B2  and  appears  as 
light  2  is  turned  up  and  1  down.) 

Hypnotist  snaps  fingers  and  awakens  Charles,  drops  curtain, 
bows,  and  retires. 

Show  4.   The  transmutation  of  metals. 

Hypnotist :  "Ladies  and  Gentlemen,  those  of  you  who  know 
the  history  of  the  sciences  know  that  all  through  the  ages  and 
down  to  the  present  time,  scientists  have  been  trying  to  change 
the  base  metals  into  the  noble  metals,  —  lead  into  silver,  iron 
into  gold,  and  so  on.  All  such  attempts  have  previously  failed, 
but  I  wish  to  announce  modestly  to-night  that  I  have  succeeded, 
with  the  help  of  my  marvelous  hypnotic  power.  I  will  now  prove 
this  to  you  by  changing  iron  into  other  metals.,, 

(Raises  curtain  showing  a  flatiron  —  or  any  iron  object  — 
on  a  box.  He  makes  passes  at  the  iron  and  says)  "O  spirit  of 
iron,  depart!  O  spirit  of  silver,  come!  Come!  Come !"  (The 
iron  slowly  changes  to  a  silver  cake  basket,  or  any  object  of 
silver.)  (The  iron  object  is  on  a  box  at  B2,  the  silver  object  is 
on  an  exactly  similar  box  at  A2.  At  first  light  2  is  up,  the  iron 
disappears  and  the  silver  appears  as  2  is  turned  down  and  1  up.) 

Put  a  gold  object  in  place  of  the  iron,  and  change  silver  to 
gold,  and  so  on. 

Show  5.  To  hatch  an  egg.  Have  an  egg  on  one  box  and  a 
chicken  on  the  other,  and  slowly  change  the  egg  to  a  chicken. 
It  is  even  funnier  if  you  have  a  full-grown  hen.  Pretend  that 
your  power  is  not  strong  enough,  great  as  it  is,  to  change  the 
hen  back  to  the  egg. 

Show  6.    Astral  bodies. 

Hypnotist :  "Ladies  and  Gentlemen,  I  will  next  give  you  an 
exhibition  of  occultism,  and  I  will  show  you  the  results  of  a 
marvelous  discovery  I  have  made.    I  have  discovered  a  liquid 


48 


GILBERT  BOY  ENGINEERING 


with  remarkable  powers.  If  a  person  drinks  this  liquid  he  is 
immediately  changed  to  his  astral  body.  This  body  appears  to 
the  eye  to  be  the  same  as  ever,  but  it  is  composed  of  bound  ether 
only  and  has  no  substance.  I  may  say  that  this  has  nothing 
whatever  to  do  with  hypnotism  ;  the  effects  are  produced  entirely 
by  the  liquid. " 

(Raises  curtain,  disclosing  Charles  and  Henry  apparently 
seated  side  by  side  with  a  glass  of  liquid  —  water  or  milk  —  in 
front  of  each.  Charles  is  at  Al  and  Henry  is  at  B3.  Both 
lights  are  up.) 

Hypnotist :  "You  now  see  Charles  and  Henry.  Boys  !  Drink 
some  of  the  powerful  liquid. "  (The  boys  do  so.)  "Now,  Ladies 
and  Gentlemen,  the  boys  appear  to  you  the  same  as  before,  but 
they  are  not. 

"Charles !  Put  your  hand  gently  through  Henry."  (Charles 
does  so.)  "Henry!  Do  you  feel  anything?"  (Henry  shakes  his 
head  to  indicate,  no.) 

"Henry !  Put  your  hand  gently  through  Charles."  (Henry  does 
so.)  "Charles!  Do  you  feel  anything?"  (Charles  moves  lips.) 
"You  say  3^011  don't  feel  anything,  but  you  wish  he  would  wash 
his  hands." 

Hypnotist :  "You  see,  Ladies  and  Gentlemen,  their  bodies  have 
no  substance.  They  are  simply  astral  bodies  made  up  of  bound 
ether.    I  will  prove  this  further. 

"Charles!  Slice  Henrygently with  the  butcher's  knife." (Charles 
does  so.)  "Henry!  Does  it  hurt?"  (Henry  moves  lips.)  "What! 
You  like  it?"  (Henry  nods  yes  and  moves  lips.)  "You  like  it  be- 
cause it  makes  you  feel  like  a  sliced  orange?"  (Henry  nods,  yes.) 

"Henry!  Chop  Charles  gently  with  a  hatchet."  (Henry  does 
so.)  "Charles!  Does  it  hurt  you?"  (Charles  shakes  head  and 
moves  lips.)  "It  doesn't  hurt  you  and  you  like  it  because  it  makes 
you  feel  like  minced  meat?"    (Charles  nods,  yes.) 

Hypnotist;  "Now,  Ladies  and  Gentlemen,  I  will  show  you 


GILBERT  LIGHT  EXPERIMENTS  49 


another  evidence  of  the  marvelous  power  of  this  liquid.  I  will 
have  Charles  pour  some  of  the  liquid  on  an  apple  and  thereby 
turn  it  into  an  astral  apple.   He  will  then  give  it  to  Henry  to  eat. 

"Charles !  Change  the  apple  and  give  it  to  Henry."  (Charles 
changes  apple  by  pouring  a  little  liquid  on  it  out  of  the  glass,  but 
instead  of  giving  it  to  Henry  he  starts  eating  it  himself.  Henry 
objects  and  apparently  knocks  the  apple  out  of  Charles's  hand. 
They  sit  in  their  chairs  and  each  punches  many  times  to  the  right. 
Their  fists  go  right  through  each  other.) 

Hypnotist  drops  curtain  and  apologizes  solemnly  to  the  audi- 
ence, saying  that  he  is  sorry  the  astral  bodies  got  beyond  his 
control.    He  bows  and  retires. 

Show  7.    Power  of  the  will  over  supernatural  beings. 

Hypnotist:  "Ladies  and  Gentlemen,  I  will  now  conclude  the 
entertainment  of  the  evening  by  giving  an  exhibition  of  the  power 
of  the  human  will  over  supernatural  beings.  Charles  has  just 
had  a  terrifying  experience  and  I  am  going  to  help  him  out." 

(Raises  curtain  and  shows  Charles  seated  at  Bl.  Charles 
is  frightened  and  keeps  looking  over  first  one  shoulder  then 
the  other.) 

Hypnotist :  "Now,  Charles,  calm  yourself  and  tell  us  exactly 
what  happened."  (Charles  moves  lips.)  "You  say  you  just  saw 
a  ghost  up  the  dark  road  near  Fred's  house."  (Charles  nods  and 
moves  lips.)  "Did  you  run?"  (Charles  nods,  yes.)  "Were  you 
afraid?"  (Charles  shakes  head,  no.)  "Why  did  you  run  then?" 
(Charles  moves  lips.)  "Oh,  you  just  wanted  to  see  whether  you 
could  beat  him  running?"  (Charles  nods,  yes.)  "Did  you  beat 
him?"  (Charles  nods,  yes.)  "Where  did  you  leave  the  ghost?" 
(Charles  moves  lips  and  waves  hand  toward  door.)  "You  left  it 
at  the  front  door  ?"  (Charles  nods,  yes.)  "Can  it  get  in  ?"  (Charles 
shakes  head  and  moves  lips.)  "Oh,  you  locked  the  door.  Well, 
it  doesn't  matter  because  you  aren't  afraid  of  ghosts,  anyway." 
(Ghost  gradually  appears  beside  Charles.    Henry,  covered  with 

K  — 4 


50 


GILBERT  BOY  ENGINEERING 


sheet,  is  seated  at  A3  and  appears  at  B3  as  light  1  is  turned  up.) 

(Charles  is  much  startled,  strikes  at  ghost  with  fists,  then  with 
knife,  drops  knife  with  a  clatter,  takes  up  hatchet  and  strikes  at 
ghost ;  Charles  is  much  agitated.  Ghost  is  calm  all  through  this  ; 
it  just  looks  at  Charles,  but  now  it  moves  over  into  Charles. 
(Henry  moves  from  chair  A3  to  Al.)  Charles  claws  at  his  own 
neck,  trying  to  tear  out  ghost.) 

Hypnotist  now  calls  out,  "Charles,  calm  yourself!  I  will 
help  you.  You  cannot  get  rid  of  the  ghost  because  it  is  your  own 
ghost,  but  now  just  sit  steady  and  I  will  pin  the  ghost  to  the 
chair  by  my  will  power  and  when  I  say,  'Come !'  get  up  quietly 
and  come  out  here  in  front." 

(Hypnotist  looks  intently  at  ghost,  makes  passes,  and  says 
quietly)  "Come !"  (Charles  comes  out  in  front.) 

Hypnotist :  "Now,  Charles,  look  at  your  own  ghost.  Do  you 
want  to  get  rid  of  him  entirely?" 

Charles:  "Yes." 

Hypnotist:  "All  right.  Now  watch  quietly  and  I  will  send 
him  away."  (Looking  at  ghost  and  pointing  finger  at  him)  "O 
ghost  of  Charles,  disappear  and  never  come  back!  Disappear! 
Disappear!"  (slowly).  (Ghost  disappears  as  light  1  is  turned 
down.) 

Hypnotist:  "Ladies  and  Gentlemen,  this  concludes  our  enter- 
tainment for  this  evening.  Thanking  you  all  for  your  kind  atten- 
tion, I  bid  you  good-night."    (Bows  and  retires.) 

ELEVATIONS 

The  illusion  show  (Fig.  73)  has  a  sheet  of  plate  glass  GG 
at  an  angle  of  45°.  You  can  put  on  a  similar  show  by  turning 
your  box  on  its  side.  You  can  make  a  boy  appear  to  rise  in 
air  and  stay  there.  The  boy  is  lying  on  his  back  on  a  rug  in 
place  of  T  and  has  his  legs  folded  as  though  he  were  sitting 
with  crossed  legs.    The  hypnotist  then  makes  proper  passes  and 


GILBERT  LIGHT  EXPERIMENTS  51 


Fig.  73.    Illusion  show 
Permission  of  Hurst  and  Company,  publishers  of  children's  books  and  toys 

commands  him  to  rise.  Two  concealed  boys  at  T  pull  the  rug  and 
the  boy  appears  to  rise.  He  can  then  be  turned  upside  down  and 
back  again.    You  can  repeat  with  the  boy  lying  down. 

FUN  BY  DAY  OR  NIGHT  WITH  TWO  MIRRORS 
Experiment  No.  41.    Magic  money.    Stand  the  two  mirrors 

vertically  on  the  table  sidewise  to  a  good  light  and  place  a  coin 

between  them.  Look 

over  each  mirror  in 

turn   into  the  other 

(Fig.  74).    Have  you 

multiplied  your  mon- 
ey wonderfully? 

Experiment  No.  42. 

Magic  lights.  Repeat 

the  above  in  the  dark 

with  a  lighted  candle 

between  the  mirrors  Fig,  74.  You  see  many  coins 


52 


GILBERT  BOY  ENGINEERING 


(Fig.  75).  Do 
you  find  many, 
many  lights? 

E  x  p  e  r  i  ment 
No.  43.  Magic 
army.  Put  a 
number  of  lead 
soldiers  on  a  nar- 
row strip  of  pa- 
per and  draw 
them  between  the 


Fig.  75.   You  see  many  lights  vertical  mirrors 

(Fig.  76).  Do  you 
see  an  immense  army  marching  in  perfect  order? 

Experiment  No.  44.  Magic  dancers.  Cut  out  of  paper  or 
cardboard  a  small  figure  of  a  man  dancing.  Attach  him  to  a 
string  and  make  him  dance  between  the  mirrors  in  a  good  light 
(Fig.  77).  Do  you  find  a  multitude  of  dancers  who  keep  time 
perfectly  ? 

Experiment  No.  45.  Magic  silver  or  copper  mine.  Separate 
the  mirrors  by  two  blocks,  place  them  one  above  the  other  and 
face  to  face  (Fig.  78)  ; 


place  a  silver  or  cop- 
per coin  on  the  lower 
mirror.  Do  you  find 
yourself  looking 
down  into  a  very 
deep  hole  with  many 
silver  or  copper  coins 
in  it? 

Why  you  see 
Many  Images  in  Par- 


allel   Mirrors.      YOU  Fig.  76.    You  see  an  army 


GILBERT  LIGHT  EXPERIMENTS  53 


Fig,  77.    You  see  many  dancers 


see  many  images 
between  two  par- 
allel mirrors  be- 
cause the  image 
formed  in  one 
mirror  is  an  ob- 
ject in  the  other, 
and  so  on. 

In  Fig.  79,  two 
mirrors,  A  and  B, 
4  inches  apart  are 
facing  each  other 
and  a  candle  be- 
tween them  is  1  inch  from  B  and  3  inches  from  A. 

In  B  the  image  Bl  is  formed  1  inch  behind  B  and  in  A  the 
image  Al  is  formed  3  inches  behind  A. 

Now  image  Al  is  7  inches  in  front  of  B  and  it  forms 
an  image  B2  7  inches  behind  B;  similarly  image  Bl  is  5  inches 
in  front  of  A  and  forms  an  image  A2  5  inches  behind  A. 
Again,  A2  is  9  inches  in  front  of  B  and  forms  an  image  B3 
9  inches  behind  B,  and  so  on. 

You  see  many  im- 
ages because  the  light 
which  enters  your 
eyes  has  been  reflect- 
ed one  or  more  times. 

If  you  are  looking 
at  Bl,  the  light  which 
enters  your  eye  ap- 
pears to  come  from 
Bl,  but  it  comes  from 

the  candle  and  is  re- 
Fig.  78.   You  look  into  a  very  deep  hole  containing      n         .  - 

much  money  fleeted   from  B. 


54  GILBERT  BOY  ENGINEERING 


!  !  .         !  !  !  !  !  !  •  1 

A2        Ai  Bi  B2  Ba 

If  you  are  looking  at  B2,  the  light  appears  to  come  from  B2, 
but  B2  is  an  image  of  Al,  and  the  light  goes  from  the  candle 
and  is  reflected  twice  before  it  enters  your  eye. 

Image  B3  is  an  image  of  A2,  which  in  turn  is  an  image  of  Bl, 
and  you  see  B3  by  means  of  light  which  has  been  three  times 
reflected.    Similarly  you  would  see  B4,  BIO,  and  B50  by  means 

of  light  reflected  4,  10, 
and  50  times. 

It  is  good  practice 
to  locate  the  images  in 
parallel  mirrors  and  to 
trace  the  paths  of  the 
light. 

Why  the  Images 
become  dim.  The  im- 
ages become  dimmer 
Fig.  80.  You  see  over  the  book  the  farther  they  are 


GILBERT  LIGHT  EXPERIMENTS  55 


Fig.  81.   You  see  over  your  head 


away :  first,  because 
some  light  is  absorbed 
by  the  mirrors  at  each 
reflection ;  and,  second, 
because  the  light  has 
traveled  a  long  dis- 
tance in  being  reflect- 
ed back  and  forth  be- 
tween the  mirrors. 

Experiment  No.  46. 
The  trench  periscope. 
To  illustrate  how  the  periscope  works,  look  over  the  top  of  a  tall 
book  as  shown  in  Fig.  80.  Place  one  mirror  against  the  book  at 
an  angle  of  45°  and  hold  the  second  mirror  above  the  book  at 
the  same  angle.  Can  you  see  over  the  top  easily  without  being 
seen  yourself?  Turn  the  upper  mirror  until  it  looks  backward 
(Fig.  81).  Can  you  see  back  over  your  head,  but  is  everything  up- 
side down?  Turn  the  upper  mirror  until  it  looks  sidewise(Fig.  82). 
Can  you  see  things,  but  are  they  turned  on  their  sides? 

THE  "WHY"  OF  THE  PERISCOPE 

Now  let  us  see 
why  the  image  is 
right  side  up  in 
some  cases  and 
not  in  others. 

The  mirrors  in 
the  regular  peri- 
scope are  parallel 
to  each  other,  and 
you  can  locate  the 
image  in  each  mir- 

Fig.  82.   You  see  things  at  the  side  ror  in  turn  as  you 


56 


GILBERT  BOY  ENGINEERING 


Let  the  arrow,  Fig. 
83,  represent  the  ob- 
ject; its  image  in  A  is 
Al  and  the  top  and 
bottom  of  Al  are  as 


did  in  the  case  of 
parallel  mirrors. 


far  behind  the  mirror 
extended  as  the  top 
and  bottom  of  the  ar- 
row are  in  front. 


Now  let  us  suppose 
mirror  B  to  be  extend- 


ing. 83.    The  "why"  of  the  periscope 


ed  as  shown  by  the  dotted  line,  then  Bl  is  the  image  of  Al  in 
this  extended  mirror  and  the  top  and  bottom  of  Bl  are  as  far 
behind  B  as  the  top  and  bottom  of  Al  are  in  front  of  B,  and 
therefore  Bl  is  right  side  up. 

In  the  second  case,  the  mirrors  are  at  right  angles  (Fig.  84). 
Al  is  the  image  of  the  arrow  in  A  extended  and  Bl  is  the  image 
of  Al  in  B  extended;  Al  is  on  its  side  and  Bl  is  inverted  for  the 
reasons  given 

above.  Ai 


In  the  third  case, 
the  image  is  on  its 
side  in  the  upper 
mirror,  and  since 
the  lower  mirror  is 
parallel  to  this  im- 
age, the  image  in 
the  lower  mirror  is 


\  ARROW 


V 


/ 


No.  47.  To  make  a 


Fig.  84.   Why  things  are  upside  down 


\ 


GILBERT  LIGHT  EXPERIMENTS  57 


trench  periscope.  Get  a  block  of 
wood  4"  X  4"  X  6",  measure  down 
1  inch  from  each  end  and  draw 
a  line  across  diagonally.  This  line 
will  be  at  45°  to  the  length  of 
the  block.  Cut  the  block  through 
on  this  diagonal  line,  see  right  side 
Fig.  85. 

Now  attach  a  mirror  to  each  diag- 
onal face  by  means  of  tacks.  Cut 
a  piece  of  stiff  cardboard  17  inches 
wide  and  as  long  as  you  wish  to 
make  the  periscope.  Tack  this  to  the 
block,  overlapping  1  inch  on  one 
side.  Paste  the  overlapping  parts 
together.  Cut  a  hole  3"  X  3"  oppo- 
site the  upper  mirror  and  a  hole 
2"  X  2"  opposite  the  lower  mirror,  and  your  periscope  is  finished. 

You  can  use  this  periscope  in  your  trench  battles ;  also  you 
can  use  it  on  a  train  to  see  forward  without  putting  your  head 
out  of  the  window.  In  this  case,  however,  you  should  fasten  the 
window  glass  over  one  of  the  holes  to  keep  cinders  out  of  your  eyes. 


Fig.  85.  The  completed  periscope 
and  the  block  used  in  it 


Fig.  86.    You  see  four  candles 


FUN   WITH  MIR- 
RORS AT  DIFFER- 
ENT ANGLES 
Experiment  No.  48. 

Mirrors  at  different 
angles.  Stand  the  mir- 
rors vertically  and  at 
right  angles  on  the 
table  (Fig.  86)  and 
place  a  lighted  candle 


58 


GILBERT  BOY  ENGINEERING 


between  them.  Do  you  see  four  candles,  the  real  candle  and 
three  images  ? 

Make  the  angle  60°.  Do  you  see  six  candles,  the  original 
candle  and  five  images? 

Make  the  angle  45°.    Do  you  see  eight  candles? 

Make  the  angle  30°.    Do  you  see  twelve  candles? 

There  are  360°  in  a  complete  circle,  and  the  number  of  candles 
you  see  in  each  case  is  360  divided  by  the  angle  between  the 

360 

mirrors.    For  example,  when  the  angle  is  90°,  you  see         or  4 

360  90 

candles ;  and  when  the  angle  is  60°,  )^ou  see   ■  or  6  candles ; 

A  60 

and  so  on. 

Experiment  No.  49.  A  one-boy  crowd.  Stand  the  mirrors 
at  90°  and  put  your  face  close  to  the  mirrors.  Are  there  four 
of  you,  yourself  and  three  images? 

Repeat  with  the  mirrors  at  the  angles  mentioned  above.  Do 
you  find  yourself  a  crowd  all  in  a  circle? 

Experiment  No.  50.  Arrows.  Stand  the  mirrors  at  90°  on  a 
piece  of  white  paper  and  draw  an  arrow  pointing  at  one  of  the 
mirrors.  Do  some  of  the  arrows  point  in  one  direction  and 
some  in  the  opposite  direction?  Keep  one  mirror  in  such  a  posi- 
tion that  the  arrow  points  directly  at  it  and  move  the  other 
mirror  until  the  angle  is  60°.  Do  the  six  arrows  point  toward 
each  other  in  pairs? 

Repeat  with  the  mirror  at  the  other  angles  mentioned  above. 

Experiment  No.  51.  An  infinite  number  of  candles.  Light 
a  candle  and  stand  the  mirrors  close  to  it  and  gradually  make 
them  parallel.    Do  you  see  very,  very  many  candles? 

When  the  mirrors  are  parallel  the  angle  between  them  is 
360 

0°,  and           is  infinity,  so  you  should  see  an.  infinite  number  of 

0 


GILBERT  LIGHT  EXPERIMENTS  59 


I 

I 

--h 

I 
i 

A2  B£ 


-   2"  — 

II" 

4- 
I 

l 


Fig.  87.    Tho  "why"  of  mirrors  at  right  angles 


images.  You  cannot, 
because  some  light  is 
lost  at  each  reflection 
and  finally  all  is  lost. 

Experiment 
No.  52.  To  locate  the 
images  in  mirrors  at 
an  angle.  Draw  two 
lines  4  inches  long  at 
right  angles  to  rep- 
resent two  mirrors  at 
right  angles  (Fig.  87) 
and  extend  them 
backward  by  dotted 

lines  to  represent  the  extended  mirrors.  Place  a  dot  1  inch  from 
A  and  2  inches  from  B,  then  image  Al  will  be  1  inch  behind  A 
and  image  Bl  2  inches  behind  B.  The  third  image  A2B2  is  an 
image  of  both  Al  and  Bl ;  it  is  1  inch  behind  A  extended  and 
2  inches  behind  B  extended. 

It  is  harder  to  locate  the  images 
when  the  angle  is  60°  or  smaller,  but 
it  will  help  you  to  know  that  the 
images  are  always  all  on  the  circum- 
ference of  a  circle  of  which  the  angle 
of  the  mirrors  is  the  center. 

Practice  locating  the  images  in 
mirrors  at  60°. 

Experiment  No.  53.  The  kaleido- 
scope. The  kaleidoscope  (Fig.  88) 
consists  of  two  mirrors  at  an  angle 
of  30°  in  a  tube  which  has  an  eye 
opening  at  one  end  and  at  the  other 
Fig.  88.  The  kaleidoscope        a  chamber  containing  pieces  of  col- 


60  GILBERT  BOY  ENGINEERING 


ored  glass.  When  you 
look  through  the  tube 
and  revolve  it,  the 
colored  pieces  of  glass 
make  beautiful  twelve- 
sided  figures  by  mul- 
tiple reflection. 

Illustrate  the  work- 
ing of  the  kaleidoscope 
as  follows  :  Draw  two 


Fig.  89.   You  see  many  twelve-sided  figures  lines   at    an   angle  of 

30°  on  a  piece  of  white  paper.  Stand  the  mirrors  on  a  block  above 
these  lines  with  the  angle  toward  a  good  light  (Fig.  89).  Now 
put  pieces  of  colored  paper  and  other  small  objects  on  a  strip 
of  paper  and  draw  the  paper  under  the  angle,  while  you  look 
down  between  the  mirrors  with  your  eye  near  the  angle.  Do 
you  see  a  series  of  twelve-sided  figures? 

ILLUSIONS 


The  Sphinx.  This 
illusion  shows  an 
Egyptian  head  with- 
out a  body  (Fig.  90). 
The  hypnotist  shows 
the  audience  an  empty 
box  with  a  glass  front. 
He  closes  and  locks 
the  door  over  the 
front,  places  the  box 
exactly  on  the  center 
of  the  table,  unlocks 
it,  opens  the  door,  and, 


behold,    there    is     an  Fig.  90.    The  Egyptian  head 


GILBERT  LIGHT  EXPERIMENTS  61 


CURTAINS 


PLAN 


Egyptian  head  in  the 
box.  The  hypnotist 
stands  near  the  audi- 
ence and  addresses 
the  head.  "O  an- 
cient Sphinx,  awake ! 
Awake!  Awake  I" 
The  sphinx  slowly 
opens  its  eyes  and 
stares  straight  ahead. 
The  hypnotist  then 
addresses  questions 
to  it  and  it  answers 
in  very  deep  and  very 
dead  tones,  and  so  on. 
Finally  the  hypnotist 
locks  the  box,  brings 
it  forward  to  the  au- 
dience, opens  it,  and 
there  is  nothing  in  it 
butahandfulof  ashes. 

The  mechanism  of  this  illusion  is  illustrated  in  Fig.  91.  The 
table  is  on  three  legs,  A,  B,  C,  with  mirrors  at  60°  between  A,  C, 
and  A,  B.  The  curtains  at  the  back  and  sides  are  exactly  alike, 
and  to  the  audience  the  images  of  the  side  curtains  appear  to  be 
the  back  curtain,  and  the  space  under  the  table  appears  quite 
empty. 

Cabinet  of  Proteus.  The  performer  puts  his  assistant  into 
the  cabinet  (Fig.  92),  closes  the  doors  a  moment,  makes  passes, 
open  doors  (Fig.  93),  and  the  assistant  is  gone.  Closes  doors 
again,  makes  passes,  opens  doors,  and  out  comes  an  entirely 
different  man.  Closes  doors  again,  makes  passes,  opens  doors, 
and  out  comes  a  lady.     Closes  doors  again,  makes  passes, 


Fig.  91.    Plan  of  the  table 


62 


GILBERT  BOY  ENGINEERING 


opens  doors,  and  out  comes  assistant. 

To  the  audience,  the  cabinet  appears 
entirely  empty  except  for  a  post,  C,  with 
a  strong  light  at  the  top.  There  are, 
however,  two  hinged  mirrors,  ab  and 
ab,  Fig.  94,  at  an  angle  of  60°  and  the 
post  covers  the  angle.  The  sides  and 
back  are  exactly  alike  and  the  images 
of  the  sides  in  the  mirrors  appear  to  the 
audience  to  be  the  back.  The  man, 
lady,  and  as- 


Fig.  92.    Cabinet  closed 
From  Hoffman's  Modern  Magic 


sistant,  of 
course,  hide 
behind  the  mirrors.  Members  of  the 
audience  stand  behind  and  beside  the 
cabinet  all  through  the  performance. 
The  assistant  swings  the  mirrors 
against  the  sides  before  he  comes  out 
the  last  time,  and  then  members  of  the 
audience  are  asked  to  examine  the  cab- 
inet, when,  of  course,  they  find  nothing. 


^^^^^^ 


Fig.  94.    Plan  of  cabinet 
From  Hoffman's  Modern  Magic 


Cabinet  open 
From  Hoffman's  Modern  Magic 


Illusion  Show.  Pharaoh's 
thumb.  Make  a  table  out  of 
cardboard  (Fig.  95)  and  stand  it 
on  three  legs,  each  of  which  is 
exactly  5%  inches  from  the  other 
two,  and  place  your  two  mir- 
rors between  A  and  B  and  A  and 
C.  Surround  it  by  screens  on 
three  sides,  making  the  sides  and 


GILBERT  LIGHT  EXPERIMENTS  63 


back  exactly  alike 
and  exactly  the  same 
distance  from  the 
table. 

Now  have  an  as- 
sistant put  his  arm 
through  a  hole  in  the 
back  curtain  and  put 
his  blackened  thumb 
up  through  a  hole  in 
the  table  top,  and  you 
are  ready  to  begin 
the  act. 

_      ,  .  .  Fig.  95.    Pharaoh's  thumb 

Explain  to  the  au- 
dience that  you  have  succeeded  in  bringing  to  life  the  thumb 
of  an  ancient  pharaoh  by  your  hypnotic  power.  Explain  that 
the  thumb  was  lost  in  battle,  fell  on  the  sands  of  the  desert  and 
dried  but  did  not  decompose.  This  pharaoh  was  a  great  hyp- 
notist, which  makes  it  easier  for  you  to  bring  his  thumb  back 
to  life.  Explain  also  that  the  thumb  will  answer  any  question 
about  the  future.  If  the  thumb  moves  forward  it  is,  yes ;  if  it 
doesn't  move  at  all  it  is,  no. 

Now  open  the  curtains,  address  the  thumb,  "O  Thumb  of  an 
ancient  Pharaoh,  awake!  Awake!  Awake!"  (slowly  and  with 
passes).  The  thumb  does  not  move.  You  now  ask,  "O  ancient 
and  sacred  Thumb,  will  Charles  get  his  wish?"  (Thumb  slowly 
nods,  yes.)  "O  ancient  and  sacred  Thumb,  will  Henry  get 
through  his  examinations?"  (Thumb  does  not  move.  No.)  And 
so  on. 

Vaudeville  Act.  The  acrobat.  You  can  put  on  a  short  but 
very  funny  act  with  a  mirror  (Fig.  96)  placed  at  an  angle  to  the 
audience. 


64 


GILBERT  BOY  ENGINEERING 


Fig.  96.    You  become  a  marvelous  acrobat 
From  Magical  Experiments,  published  by  David  McKay  Co.,  Philadelphia 

FUN  WITH  THE  CURVED  MIRROR 

Experiment  No.  54.  Converging  sunlight.  Open  the  slit  in 
your  darkened  room  to  its  full  size  and  allow  the  sunlight  to 
fall  on  the  concave  (curved-in)  side  of  your  curved  mirror. 
Make  a  dust.  Is  the  sunlight  converged  to  a  point  and  does  it 
diverge  beyond  this  point  (Fig.  97)  ?  This  point  is  the  focus  of 
the  mirror. 

Experiment  No.  55.  Diverging  sunlight.  Turn  the  convex 
(curved-out)  side  of  the  mirror  to  the  sunlight  (Fig.  98).  Is 
the  sunlight  reflected  and  diverged  or  spread? 


GILBERT  LIGHT  EXPERIMENTS  65 


Experiment  No.  56. 

Picture  of  the  sun. 
Remove  the  shutter, 
stand  the  mirror  on 
the  table  in  the  sun- 
light, and  focus  the 
sunlight  on  a  strip  of 
paper  inch  wide 
(Fig.  99).  Is  the  pic- 
ture of  the  sun  round 
and  very  bright? 

Experiment  No.  57.      Fig*  97,  You  see  a  brilliant  focus  in  dusty  air 
The  focus  is  very  hot.    Focus  the  sunlight  on  your  hand  with 
the  concave  mirror  (Fig.  100).  Is  it  hot?  It  is,  because  all  the 
heat  of  the  sunlight  is  concentrated  at  the  focus. 

Experiment  No.  58.  To  light  a  match  with  sunlight.  Place 
a  match  in  front  of  a  narrow  strip  of  paper  (Fig.  101)  and  focus 
the  sunlight  on  the  head.    Does  the  match  light? 

Experiment  No.  59.  A  magic  cannon.  Stick  a  needle  into 
the  under  side  of  a  cork  and  stick  a  match  on  the  other  end  of 

the  needle  (Fig.  102), 
with  a  small  piece  of 
paper  at  one  side  of  the 
head.  Insert  the  stopper 
in  an  empty  bottle,  focus 
the  sunlight  on  the 
match  head  through  the 
glass  sides  (Fig.  103). 
Does  the  match  light 
and  are  the  cork,  nee- 
dle, and  match  driven 
out   with  a  satisfactory 

You  see  the  light  spread  P°P  ? 


66 


GILBERT  BOY  ENGINEERING 


■ 

The  lighted  match 
heats  the  air  and  the 
expanding  air  drives 
out  the  cork. 

Experiment  No.  60. 
Focal  length  of  con- 
cave mirror.  Focus 
the  sunlight  on  a  nar- 
row piece  of  paper 
and  measure  the  dis- 
tance between  the 
back  of  the  mirror  and  the  paper.  This  is  the  focal  length  of 
the  mirror.    Do  you  find  it  to  be  about  two  inches  ? 

Experiment  No.  61.  Focal  length  of  convex  mirror.  Make 
two  pencil  dots  just  2  inches  apart  on  a  piece  of  cardboard  and 
between  these  punch  two  holes  just  1  inch  apart.  Hold  the 
cardboard  between  the  convex  mirror  and  the  sun  and  move  it 
until  the  light  which  passes  through  the  holes  1  inch  apart  is 
reflected  to  the  dots  2  inches  apart,  and  measure  the  distance 
from  the  back  of  the  mirror  to  the  card.    This  is  the  focal 


Fig.  99.   You  see  a  brilliant  picture  of  the  sun 


Fig.  100.   You  find  the  focus  hot 


GILBERT  LIGHT  EXPERIMENTS  67 


length  of  the  convex 
mirror.  Do  you  find 
it  to  be  2  inches? 

There  is  no  real 
focus  for  a  convex 
mirror  because  it 
spreads  the  light,  but 
the  reflected  rays  ap- 
pear to  come  from  a 
point  2  inches  behind 


i 

jj 

■ 

Fig.  102.  How  to  arrange  cork,  needle,  and  match 

the  focus  (Fig.  104). 
Do  you  find  a  small 
inverted  picture  in 
natural  colors  of  the 
window  and  of  the 
things  outside  the 
window? 

Have  a  friend 
move  about  near  the 
window.  Do  you  get 
his  picture? 


Fig.  101.    You  light  a  match  with  sunlight 

the  mirror.  An  un- 
real focus  of  this  kind 
is  called  avirtualfocus. 

Experiment  No.  62. 
Pictures.  Go  to  the 
back  of  the  room, 
turn  the  concave  mir- 
ror toward  the  win- 
dow, and  hold  a  piece 
of  paper  three-quar- 
ters inch  wide  near 


Fig.  103.   You  light  the  match  in  the  bottle 


68 


GILBERT  BOY  ENGINEERING 


Fig.  104.    You  get  a  picture 


Experiment 

No.  63.  Your  own 
image.  Look  at 
yourself  in  the  con- 
cave mirror.  Are 
you  upside  down  and 
small?  Bring  your 
eye  closer  to  the 
mirror  than  the  focus 
(2  inches)  (Fig.  105). 
Is  your  eye  large 
and  right  side  up?  Look  at  yourself  in  the  convex  side.  Are 
you  small  and  right  side  up  in  all  cases? 

THE  "WHY"  OF  THE  CURVED  MIRRORS 
Waves.    When  parallel  waves  (1),  Fig.  106,  strike  the  con- 
cave side  of  the  mirror,  they  are  reflected  and  so  curved  in  that 
they  converge  at  the  focus  and  then  diverge. 

When  parallel  waves  strike  the  convex  side  (2),  they  are 
reflected  and  so  curved  out  that  they  diverge  and  never  meet. 

Rays.  The  curved 
mirror  is  part  of  a 
sphere  and  the  cen- 
ter of  the  sphere  is 
at.  C,  Fig.  107(1).  The 
lines  CA  are  radii  of 
the  sphere  and  they 
are  perpendicular  to 
the  mirror.  When  par- 
allel rays  strike  the 
concave  mirror  they 
make    equal  angles 

with  these  perpendic-  Fig.  105,  Your  eye  is  enlarged 


GILBERT  LIGHT  EXPERIMENTS  69 


Fig.  106.    (1)  Parallel  waves  are  curved  in.    (2)  Parallel  waves  are  curved  out 

ular  radii  and  cross  at  the  focus  F.  The  line  through  the  center 
O  of  the  mirror  and  through  the  center  C  of  the  sphere  is  called 
the  principal  axis  of  the  mirror.   You  will  notice  that  the  parallel 


Fig.  107  (1).   How  parallel  rays  are  converged  by  a 
concave  lens  and  diverged  by  a  convex  lens 


Fig.  107  (2) 


Fig.  108.    A  searchlight 
Courtesy  of  the  Scientific  American 


GILBERT  LIGHT  EXPERIMENTS 


71 


Fig.  109. 


A  parabolic 
reflector 
From    Black    and  Davis' 
Practical  Physics,  published 
by  The  Macmillan  Co. 


rays  which  are  above  the  principal  axis 
before  they  strike  the  concave  mirror  are 
below  it  afterward  and  vice  versa.  This 
explains  why  the  images  you  see  in  the 
concave  mirror  are  reversed. 

When  your  eye  is  nearer  than  the 
focus,  it  intercepts  the  rays  before  they 
can  cross,  and  your  image  appears  to  be 
behind  the  mirror,  right  side  up  and 
large. 

When  parallel  rays  strike  the  convex 
side  of  the  mirror,  Fig.  107(2),  they  make  equal  angles  with  the 
radii  (CA  extended)  ;  they  diverge  but  appear  to  come  from  the 
focus  F.    This  is  the  unreal  or  virtual  focus. 

The  rays  above  the  principal  axis  before  reflection  are  above 
it  afterward,  and,  therefore,  the  images  in  the  convex  mirror  are 
right  side  up. 

Searchlight  Reflectors.  The  reflectors  on  battleship  search- 
lights (Fig.  108)  are  made  in  the  shape  of  a  parabola  (Fig.  109). 
Parallel  rays  which  strike  parabolic  reflectors  converge  exactly 
at  the  focus,  and  conversely  if  a  light  is  placed  exactly  at  the 
focus  the  reflected  light  consists  of  parallel  rays  which  go 
straight  forward.  The  reflectors  on  automobile  and  locomotive 
headlights  are  also  parabolic,  and  the  lamp  is  placed  at  the  focus. 
Spherical  Aberration.  Spherical  mirrors  do  not  converge  all 
parallel  rays  at  the  focus  because  those 
which  strike  near  the  edge  are  reflected  be- 
hind the  focus  (Fig.  110).  This  is  called  the 
spherical  error  or  spherical  aberration  of 
the  mirror.  Conversely  if  a  light  is  placed 
at  its  focus  a  spherical  mirror  does  not  re- 
F%he1r0icai:aberi^tioTi  °f  fleet  it  in  parallel  rays.  This  explains  why 
^S<cnTp^sic*"pM6M«2ed     *t  is  not  used  as  a  first-class  reflector, 

by  The  Macmillan  Go, 


— *  ~ — -jr. 

— »  

c'  *tftf\ 

72 


GILBERT  BOY  ENGINEERING 


REFRACTION  OF 
LIGHT 

When  light  passes  in  a 
slanting  direction  from  one 
medium  to  another,  —  for  ex- 
ample, from  air  to  water 
or  the  reverse,  or  from  air 
to  glass  or  the  reverse, — 
part  of  it  is  reflected 
at  the  surface  between  the 
two  media  and  part  of  it 
enters  the  second  medium  but 
is  bent  out  of  its  path,  from  ABC  to  ABD,  Fig.  111.  This  bending 
is  called  refraction.  When  light  passes  from  air  to  any  denser 
medium  as  water  or  glass,  it  is  bent  toward  a  line  NN  drawn 
perpendicularly  through  the  surface  at  the  point  it  enters.  See 

Fig.  112  (1).  When  light 
passes  from  water  or  glass  to 
air,  it  is  bent  away  from 
the  perpendicular  NN.  See 
Fig.  112  (2). 


Fig.  111.    The  light  is  bent  or  refracted 
From  Lynde's  Physics  of  the  Household, 
published  by  The  Macmillan  Co. 


Fig.  112  (1).  Part  of  the 
light  is  reflected  and  part 
refracted 


Fig.  112  (2) 


GILBERT  LIGHT  EXPERIMENTS  73 


Fig.  113.    You  see  light  bent  toward  the  perpendicular  NN 

FUN  WITH  SUNLIGHT 
Experiment  No.  64.  Air  to  water.  Allow  a  beam  of  sunlight 
to  pass  through  the  slit  in  your  darkened  room.  Cut  a  slit  1  inch 
long  and  %  inch  wide  in  a  piece  of  cardboard,  put  this  over  your 
mirror,  and  reflect  sunlight  into  a  glass  pitcher  full  of  water 
into  which  you  have  put  2  or  3  drops  of  milk  (1),  Fig.  113.  Vary 
the  slant  of  the  beam  of  sunlight  which  strikes  the  water  and 
view  the  beam  in  the  water  through  the  sides  of  the  pitcher.  Is 
some  of  the  light  reflected  at  the  surface  of  the  water?  Does 
some  of  it  enter  the  water  and  is  it  bent  or  refracted?  Make 
the  beam  split  on  the  side  of  the  pitcher  so  that  half  is  inside 
and  half  outside.  Is  the  beam  in  the  water  bent  toward  an  im- 
aginary perpendicular  at  the  point  it  enters?  Repeat  this  with  a 
glass  of  milky  water  (2).  Repeat  with  a  bottle  of  milky  water  (3). 
Use  a  bottle  with  flat  sides. 


74 


GILBERT  BOY  ENGINEERING 


Fig.  114.    You  see  part  of  the  light  reflected  and 
part  refracted 


E  xper  iment 
No.  65.  Air  to  glass. 
Make  the  beam  split 
on  the  thick  glass 
plate  standing  on  its 
edge,  on  two  match- 
es, on  paper  (Fig. 
114).  You  cannot 
see  the  light  in  the 
glass  but  you  can 
see  it  on  the  paper 
below  after  it  has 
passed  through  the 

glass.  Is  the  light  which  passes  through  the  glass  bent  toward  an 
imaginary  perpendicular  NN  drawn  at  the  point  it  enters? 

Let  the  sunlight  enter  through  a  slit  1  inch  long  and 
%  inch  wide.  Split  the  beam  of  light  on  the  edge  of  the 
glass  plate  and  hold  a  piece  of  paper  behind  the  plate.  Tilt 
the  plate  to  different  angles.  Is  the  light  which  passes  through 
the  glass  plate  always  bent  toward  the  perpendicular  NN? 

Experime  n  t 
No.  66.  A  glass  of 
water.  Remove  your 
shutter  and  stand  a 
glass  of  water  in 
sunlight  near  the 
window ;  fill  the 
glass  to  the  top  and 
put  paper  around 
the  sides  to  keep 
out  the  sunlight.  Is 
the  sunlight  which 

Strikes     the     water  Fig.  115.    The  light  is  bent  down 


 GILBERT  LIGHT  EXPERIMENTS  75 

surface  bent  down,   as   shown  in  ^         \n  Smooth 

Fig.  115?  yS\S/\bGrowi& 
Explanation   of  Refraction.  A 

beam  of  sunlight  is  bent  or  re- 
fracted when  it  passes  from  air  to 
water  because  light  travels  more 
slowly  in  water  than  it  does  in  air.  ^Lond 
Its  velocity  in  water  is  only  three- 
fourths  of  its  velocity  in  air.  Fig  11G  The^hyv  of  refracti0n 

"Mrkw  +r*  cpp  +hp>  rnnnprtinn  hp-  From  the  Ontario  High  School 
JNOW  tO  See   tne   Connection    De-     PhysicSj    by    permission    of  the 

tween  change  in  direction  and  change  publishers 
in  velocity, let  us  consider  what  would  happen  if  a  regiment  of  soldiers 
marched  in  a  slanting  direction  BD  from  smooth  ground  to  rough 
ground,  as  shown  in  Fig.  116.  The  men  would  march  less  rapidly  on 
the  rough  ground  and  the  direction  of  the  marching  lines  would  be 
changed.  The  line  AB  is  still  on  smooth  ground  and  is  straight.  Part 
of  the  line  ab  is  on  rough  ground  and  this  part  is  somewhat  behind. 
The  line  cd  has  a  larger  part  on  rough  ground  and  this  part  is  behind. 
The  line  CD  is  wholly  on  rough  ground  and  it  is  marching  in  a  direc- 
tion DE  different  from  BD,  and  it  would  continue  in  this  new  direc- 
tion. This  is  exactly  what  happens  to  parallel  light  waves.  They  are 
bent  toward  the  perpendicular  when  they  pass  at  a  slant  from  air  to 
water  or  glass  because  they  travel  more  slowly  in  water  or  glass  than 
they  do  in  air.  They  are  bent  away  from  the  perpendicular  when 
they  pass  at  a  slant  from  water  or  glass  to  air  because  they 
travel  faster  in  air  than  they  do  in  water  or  glass. 

REFRACTION  OF  SPHERICAL  WAVES 
Experiment  No.  67.  A  coin  under  water.  Put  a  coin  in  a  glass 

of  water  and  look  down  at  it  through  the  water  (Fig.  117).  Does 

it  appear  to  be  nearer  than  it  really  is? 

You  see  the  coin  because  light  passes  from  it  to  your  eyes. 

This  light  is  in  the  form  of  spherical  waves  in  the  water,  but 


76 


GILBERT  BOY  ENGINEERING 


when  these  waves 
enter  the  air  they 
become  more 
curved  (see  BC, 
Fig.  118)  because 
the  parts  which  en- 
ter the  air  first 
travel  faster  and  get 
ahead  of  the  parts 
still  in  water. 

Now  your  eye 

Fig.  117.   The  coin  appears  closer  to  you  than  it  is      estimates    the  dis- 
tance of  an  object 

partly  by  the  curvature  of  the  waves 
which  enter  it  from  the  object.  The 
curvature  of  the  waves  which  enter 
your  eye  from  the  coin  is  the  same  as 
though  the  coin  were  at  a  point  A 
only  three-fourths  the  depth,  and  this 
is  the  reason  the  coin  appears  to  be 
at  A. 

If  you  look  at  the  coin  in  a  slant- 
ing direction,  it  appears  to  be  nearer 
the  surface  still,  because  the  light  is 
bent  more  and  more  the  greater  the 
slant  of  the  rays  from  the  coin  to 
the  surface. 

RELATION  BETWEEN  ANGLES 
OF  REFRACTION  AND 
INCIDENCE 

If  the  light  ray  in  Fig.  119  is  pass- 

r  •  .<         .1      «>         Fig.  118.    Why  the  coin  appears 

ing  from  air  to  water,  then  the  line  closer  to  you 


GILBERT  LIGHT  EXPERIMENTS  77 


RM  is  always  exactly  three- 
fourths  the  length  of  IM  no 
matter  how  large  or  small  i 
may  be.  If  the  light  passes  in 
the  opposite  direction,  the  same 
relation  holds  until  the  critical 
angle  is  reached.  (See  page  87 
for  definition  of  critical  angle.) 

If  the  ray  is  passing  from 
air  to  glass,  RM  will  always  be 
two-thirds  of  IM,  and  this  rela- 
tion holds  if  the  light  passes 
from  glass  to  air,  until  the  criti- 
cal angle  is  reached. 

This  gives  you  the  relation  between  the  angle  of  incidence 
and  the  angle  of  refraction  in  all  cases. 


Fig.  119.  RM  is  throe-fourths  of  IM  in 
water  and  two-thirds  of  IM  in  glass 
From  Lynde's  Physics  of  the  House- 
hold, published  by  The  Macrnillan  Co. 


FUN  BY  DAY  OR  NIGHT 
Experiment  No.  68.    Magic  lead  pencil.    Put  a  pencil  in 
a  glass  of  water  in  a  slanting  direction  and  sight  along  it 

  (Fig.  120).  Does 

it  appear  to  be 
bent  up?  It  does, 
because  the  light 
from  it  is  bent 
as  shown  in 
Fig.  121. 

E  x  p  e  r  i  ment 
No.  69.  Magic 
ruler.  Put  a  ruler 
vertically  in  a 
pitcher   of  water 

Fig.  120.   You  see  a  bent  pencil  to   a   depth   of  4 


78 


GILBERT  BOY  ENGINEERING 


Fig.  121.    The  light  is  bent 
From  Black  and  Davis'  Practical  Phys- 
ics, published  by  The  Macmillan  Co. 


inches  (Fig.  122).  Does  the 
part  under  water  appear  to  be 
only  3  inches  long  when  viewed 
vertically?  It  does,  because 
light  travels  in  water  only 
three-fourths  as  fast  as  it  does 
in  air. 

Does  it  appear  much  shal- 
lower when  viewed  at  a  slant? 
It  does,  because  light  is  bent  more  the  greater  the  angle  at  which 
it  leaves  the  water. 

Experiment  No.  70.  An  elastic  ruler.  Shove  the  ruler  to 
the  bottom  of  a  pail  of  water  and  lift  it  out.  Does  it  appear  to 
stretch? 

Experiment  No.  71.  Magic  glass.  Stand  a  ruler  at  one  end 
of  the  glass  prism  held  on  one  edge,  Fig.  123  (1).  Does  the 
bottom  appear  only  two-thirds  its  real  depth  when  viewed  ver- 
tically? It  does,  because  light  travels  only  two-thirds  as  fast 
in  glass  as  it  does  in  air. 

Does  it  appear  even  shallower  when  viewed  at  a  slant?  It 
does,  because  light  is  bent  more  the  greater  the  angle  at  which 
it    leaves   the  glass. 

Repeat   this  with 
the  prism  on  end. 

Repeat  with  the 
glass  plate  on  its 
edge,  Fig.  123  (2). 

Experiment  No.  72. 
Phantom  coin.  Fill  a 
long,  deep  pan  with 
water.  Put  a  coin  on 
the  bottom  and  view 
it  vertically  and  then       Fig.  122.   You  see  the  ruler  shortened  in  water 


GILBERT  LIGHT  EXPERIMENTS  79 


Fig.  123  (1).    The  prism  and  the  glass  plate 
appear  shallower  than  they  are 


at  greater  and  greater 
slants.  Does  the  coin 
seem  to  rise?  Why? 

Experiment  No.  73. 
A  disappearing  coin. 
Stand  a  coin  on  edge 
in  a  tin  funnel  full  of 
water,  ask  a  friend  to 
stand  so  that  he  can 
just  see  the  top  over 
the  edge  of  the  funnel, 
and  then  let  the  water 
run  out.  Does  he  find 

that  he  can  no  longer  see  the  coin  from  where  he  stands?  Why? 

Experiment  No.  74.  A  broken  looking-glass.  Play  this  trick  on 
your  family.  Take  a  piece  of  soap  and  mark  a  star  with  radiating 
lines  near  one  edge  of  a  looking-glass  (Fig.  124).  The  family  will 
think  the  glass  is  broken.  A  real  break  shows  up  because  the 
light  is  refracted  at  the  break  and  this  gives  a  fair  imitation. 
Where  is  the  Fish?    The  three  boys  1,  2,  and  3  in  Fig. 

125  are  looking  at 
the  same  fish  and 
they  see  it  at  the 
three  different  posi- 
tions 1,  2,  and  3, 
because  the  light 
from  the  fish  is  bent 
more  the  greater  the 
slant  it  has  when  it 
reaches  the  water 
surface.  None  of 
them  see  the  fish 
Fig.  123  (2)  where  it  is. 


80 


GILBERT  BOY  ENGINEERING 


Fig.  124.   A  broken  mirror 


How  Deep  is  the  Water?  If  you  have  gone  swimming  in 
very  clear  water  you  know  that  it  always  looks  shallower  than 
it  is.  If  the  water  is  at  the  same  depth  everywhere  it  will  look 
to  you  shallower  in  the  distance,  for  the  reasons  given 
above. 

How  Tall  are  You  to  a  Fish?  If  you  are  6  feet  tall  a  fish 
(Fig.  126)  sees  you  as  8  feet  tall,  because  the  curved  waves  from 
you  are  made  less  curved  in  water  and,  therefore,  appear  to 
come  from  a  more  distant  point. 

Experiment  No.  75.  Breaking  a  pencil  without  touching  it. 
Look  at  a  pencil  in  a  slanting  direction  through  a  bottle  of 
water  with  flat  sides  or  through  the  edges  of  the  glass  plate. 
Does  it  appear  to  be  broken  into  three  parts?  Why? 


GILBERT  LIGHT  EXPERIMENTS  81 


Fig.  125.    They  see  the  fish  at  different  depths 


Fig.  126.  You  appear  to  a  fish  to  be  four-thirds  bigger  than  you  are 
K  —  6 


82 


GILBERT  BOY  ENGINEERING 


Fig.  127.    The  light  bends  at  A 


Experiment  No.  76.  Shifting  pin. 
Put  the  glass  plate  flat  on  a  piece  of 
paper  on  the  table.  Stick  the  pins 
A,  B  on  each  side  (Fig.  127)  and  sight 
from  pin  A  to  B  through  the  glass. 
Does  B  appear  to  be  shifted?  Draw 
lines  around  the  edges  of  the  plate, 
aim  a  ruler  at  the  two  pins  through 
the  glass,  and  draw  a  line  along  the 
ruler;  then  draw  a  line  from  B  to  A 
and  draw  a  line  perpendicular  to  the  edge  of  the  plate  at  A.  This 
shows  that  the  light  which  passes  from  B  to  A  in  glass  is  bent 
away  from  the  perpendicular  when  it  enters  air. 

Experiment  No.  77.  Shifting  line.  Put  the  plate  on  a  piece 
of  paper  (Fig.  128)  and  draw  lines  around  the  edge.  Now  draw 
a  slanting  line  AB,  sight  along  a  ruler  through  the  glass  at  this 
line,  and  draw  the  line  CD  along  the  ruler.  Is  the  line  parallel 
to  AB  but  shifted?  Draw  per- 
pendiculars at  B  and  C  and 
draw  a  line  from  B  to  C.  The 
light  from  A  passes  into  the 
glass  at  B  and  is  bent  toward 
the  perpendicular  at  B;  it  pass- 
es from  glass  to  air  at  C  and 
is  bent  away  from  the  perpen- 
dicular at  C. 

Experiment  No.  78.  Things 
are  not  where  they  seem.  Look 
at  a  lighted  candle  through 
your  glass  prism  (Fig.  129). 
Does  the  candle  appear  to  be 
in  a  different  place? 

Does  it  also  appear  to  be         Fig.  128.   The  light  bends  at  b  and  c 


GILBERT  LIGHT  EXPERIMENTS  83 


beautifully  colored  ? 
You  will  experiment 
with  colors  soon. 

Experiment  No.  79. 
Bending  light  around 
a  corner.  Stand  the 
prism  on  end  on  paper 
and  draw  a  triangle 
around  the  end  (Fig. 
130).  Now  draw  a 
line  AB  slanting  to- 


ward one  side.    Sight  Fig  129    You  gee  a  gMfted  candle 

along  a  ruler  through 

the  prism  at  this  line  and  draw  a  line  CD  along  the  ruler.  Now 
remove  the  prism,  draw  short  perpendiculars  at  B  and  C,  and 
join  BC. 

The  light  from  A  enters  glass  at  B  and  is  bent  toward  the 
perpendicular ;  it  enters  air  again  at  C  and  is  bent  away  from  the 
second  perpendicular.  This  is  why  the  light  is  bent  around  a 
corner  by  your  prism. 

Experiment  No.  80.  To  see  under  water  from  a  boat.  You 
cannot  see  things  under  water  from  air  usually,  because  the 
light  reflected  from  the  surface  blinds  you  to  the  light  coming 
from  beneath  the  surface.    You  can  easily  see  through  the  sur- 


Fig.  130.    The  light  bends  around  a  corner 


Fig.  131.    You  can  see  under  water  through  a  tube 


GILBERT  LIGHT  EXPERIMENTS  85 


Fig.  132.    Spotting  submarines 


86 


GILBERT  BOY  ENGINEERING 


face,  however,  through  a  pipe 
of  any  kind,  as  shown  in  Fig. 
131,  because  the  sides  of  the 
pipe  keep  the  reflected  light 
out  of  your  eyes.  Try  this  with 
a  pipe  2  or  3  feet  long. 

Experiment  No.  81.  To  see 
the  fish  you  are  trying  to  catch. 
If  you  can  fish  under  a  boat- 
house  or  under  a  wharf,  you 
can  see  the  fish  deep  down  under  water  because  the  house  or 
wharf  prevents  surface  reflection.  You  can  do  this  also  as  fol- 
lows :  Stretch  a  blanket  or  tarpaulin  between  two  boats  and  put 
your  head  under  it.  The  surface  reflection  is  removed  and  you 
will  be  able  to  see  to  great  depths. 

Spotting  Submarines.  Submarines  are  easily  spotted  at  great 
depths  from  a  dirigible  or  airplane  at  a  great  height  above  the 
surface  (Fig.  132)  because  at  these  great  heights  the  light  re- 


Fig.  133.    How  light  passes  from  water 
to  air 

From  Black  and  Davis*  Practical  Phys- 
ics, published  by  The  Macmillan  Co. 


Fig.  134.    You  see  a  phantom  pin 
Permission  of  Hurst  and  Company,  publishers  of  children's  books  and  toys 


GILBERT  LIGHT  EXPERIMENTS 


87 


fleeted  vertically  upward  is  not  so  great  as  the  vertical  light 
received  from  objects  beneath  the  surface. 

Total  Reflection.  When  light  passes  from  water  to  air  in 
a  slanting  direction  (Fig.  133),  part  of  it  is  reflected  and  the  part 
which  passes  through  is  bent  away  from  the  perpendicular.  As 
the  slant  becomes  greater  the  bending  is  greater,  and  finally 
the  light  which  passes  into  air  is  at  right  angles  to  the  per- 
pendicular. If  the  light  in  water  is  still  more  slanting  when 
it  reaches  the  surface,  it  does  not  pass  into  air  at  all,  but  is  all 
reflected  back  into  the  water.  This  is  called  total  reflection. 
The  angle  at  which  this  takes  place  in  water  is  any  angle  greater 
than  48.5°,  and  in  crown  glass  and  hard  flint  glass,  any  angle 
greater  than  41°  and  37°  respectively.  These  angles  are  called 
the  critical  angles  for  these  substances. 

Experiment  No.  82.  A  phantom  pin.  Cut  a  slice  of  cork, 
attach  a  pin  to  the  under  side,  float  the  cork  on  the  surface  of 
water  in  a  full  glass,  stand  the  glass  on  the  table,  and  look  at 
the  cork  from  the  level  of  the  table.  Can  you  see  a  phantom 
pin  above  the  cork  (Fig.  134)  ?  You  see  it  by  means  of  light 
reflected  from  the  under  side  of  the  water  surface. 

Experiment  No.  83.  A  broken  spoon.  Put  a  spoon  in  a  glass 
half  filled  with  water  and  look  at  the  under  side  of  the  water 
surface  through  the  side  of  the  tumbler.  Do  you  find  a  brilliant 
image  of  the  part  of  the  spoon  in  water?  You  see  this  by  light 
reflected  at  the  under  surface. 

Prism  Glass.  These  prism  glasses,  Figs.  135  (1)  and  (2), 
are  used  to  throw  light  to  the  rear  of  a  store,  or  from  the  sidewalk 
into  the  basement.  They         J  JL  t  f 


are  made  of  glass  and 
have  prisms  on  one  side. 
The  light  which  enters 
them  is  totally  reflected 
from  the  inside  surface 


Fig.  135  (1)  Fig.  135  (2) 

Light  is  reflected  from  inside  surface  of  prism 
From  the  Ontario  High  School  Physics,  by  per- 
mission of  the  publishers 


88 


GILBERT  BOY  ENGINEERING 


of  the  prisms  and  is  directed  to  the  back  of 
the  building  or  basement. 

Right-angled  Prisms.  These  are  made  of 
glass  and  act  as  mirrors,  in  some  opera  glasses 
and  other  optical  instruments.  Light  which 
enters  one  right-angled  face,  AC,  Fig.  136,  is 
totally  reflected  at  the  slanting  face  and  passes 
out  through  the  other  right-angled  face  BC. 

ATMOSPHERIC  REFRACTION 

Mirages.  A  ship  at  sea  sometimes  appears  upside  down 
(Fig.  137)  because  the  air  near  the  cold  water  is  colder  and 
denser  than  the  air  above  and  the  light  from  the  ship  is  refracted 
as  it  passes  from  each  layer  of  cold  air  to  the  warmer  layer 
above  and  is  finally  totally  reflected.  The  light  which  enters  the 
sailor's  eye  appears  to  come  from  the  image  above. 

Mirages  on  the  hot  deserts  are  caused  by  light  from  the 


Fig:.  137.    A  mirage 
From  Aldons'  Elementary  Course  of  Physics,  published  by  The  Macmillan  Co. 


Fig.   136.    A  right- 
angled  prism 


GILBERT  LIGHT  EXPERIMENTS  89 


clouds  which  passes  from  the  upper  cold  air  through  warmer 
and  warmer  lower  layers.    It  is  refracted  and  finally  totally  re- 
flected and  the  clouds  look  like  a  lake  of  water  on  the  ground. 
Sunset  and  Sunrise.    You  see  the  sun  before  it  is  up  and 


Fig.  138.    Why  you  see  the  sun  before  it  rises  and  after  it  has  set 
From  Appleton's  School  Physics,  published  by  the  American  Book  Co. 

after  it  has  set  because  light  from  it  is  refracted  by  successive 
layers  of  air  which  are  denser  the  nearer  they  are  to  the 
earth. 

The  direct  ray  SD,  Fig.  138,  could  not  be  seen  at  A  because 
the  earth  is  in  the  way,  but  the  light  SB  is  seen  because  it  is 
refracted  to  A. 


COLOR 

Spectrum.  When  a  beam 
of  sunlight  passes  through  a 
glass  prism  as  shown  in  Fig. 
139,  it  is  spread  out  into  a 
colored  band  called  the  spec- 
trum. This  spectrum  contains 
all  the  primary  colors,  of 
which  those  most  easily  rec- 
ognized are  in  order:  red, 
orange,  yellow,  green,  blue, 
indigo,  and  violet. 


Fig.  139.    A  prism  produces  a  spectrum 

from  white  light 
From  Lynde'8  Physics  of  the  Household, 
published  by  The  Macmillan  Co, 


90 


GILBERT  BOY  ENGINEERING 


Fig.  140.    A  spectrum  recombined  to  produce  white  light 


White  Light  made  up  of  all  Colors.  The  experiment  above 
shows  that  white  light  is  made  up  of  lights  of  all  primary  colors. 

This  is  proved  again 
by  passing  the  spec- 
trum through  a  prism 
turned  in  the  opposite 
direction  (Fig.  140) ; 
the  colors  are  recom- 
bined to  produce  white 
light. 

It  can  be  proved 
also  by  turning  the 
prism  back  and  forth 
quickly  (Fig.  141). 
The  colors  overlap  at 
the  center  and  pro- 
duce white  light. 

Dispersion.  You 
know  that  light  is  re- 
fracted or  bent  when 


!Fig.  141.    White  light  is  made  up  of  many 
primary  colors 
Courtesy  of  the  Scientific  American 


GILBERT  LIGHT  EXPERIMENTS  91 


Fig.  142.     A  beautiful  spectrum  by  reflection  under  water 


it  passes  from  air  to  water  or  glass  or  the  reverse,  because  it 
travels  more  slowly  in  water  and  glass  than  it  does  in  air.  Now 
the  waves  of  red  light  are  longer  than  those  of  orange,  the  waves 
of  orange  are  longer  than  those  of  yellow,  and  so  on,  the  waves 
of  each  light  beginning  at  the  red  end  of  the  spectrum  are  longer 
than  those  next  to  it  until  we  get  to  the  very  shortest,  namely, 
the  waves  of  violet  light.  It  has  been  found  by  experiment  that 
the  shorter  the  waves,  the  more  slowly  they  travel  in  water  or 
glass  and,  therefore,  the  more  they  are  refracted  or  bent  when 
they  pass  from  air  to  water  or  glass,  or  the  reverse.  When  white 
light  passes  through  a  prism  then,  the  shorter  waves  are  bent 


92 


GILBERT  BOY  ENGINEERING 


•*         ONE  WAVE  LENGTH 


\ 


or  refracted  more  than  the 
longer  waves  and  as  a  result 
the  white  light  is  spread  out 
into  the  spectrum.  This 
spreading  of  light  is  called 
dispersion. 


Pig.  143.    The  waves  destroy  each  other 
or  interfere 


INTERFERENCE 


Spectrum  by  Reflection. 

Another  beautiful  method 
of  producing  a  spectrum  is 
illustrated  in  Fig.  142.  A 
mirror  is  placed  in  a  slant- 
ing position  under  water  in 
a  pan  and  a  beam  of  sunlight 
is  allowed  to  fall  on  the  mir- 
ror. The  sunlight,  in  going 
through  the  water  to  the 


mirror  and  back,  really  passes  through  a  prism  of  water  and  it  is 
spread  out  or  dispersed  into  a  beautiful  spectrum. 

If,  after  the  spectrum  is  formed,  the  surface  of  the  water  is 
stirred,  the  colors  of  the  spectrum  are  mixed  and  the  reflected 
beam  is  white.  This  proves  again  that  white  light  is  made  up 
of  all  the  colors  of  the  spectrum. 

Interference.  In  a  water  wave  the  particles  of  water  simply 
move  up  and  down,  but  the  wave  moves  forward.  A  wave  length 
is  a  hill  and  a  hollow. 

If,  now,  two  waves  of  exactly  the  same  length  come  together 
in  such  a  way  that  one  is  one-half  wave  behind  the  other  (Fig. 
143),  the  hill  of  one  coincides  with  the  hollow  of  the  other,  the 
particles  of  water  do  not  move  at  all,  and  one  wave  destroys  the 
other.  This  is  called  interference. 

The  same  thing  occurs  in  light  waves ;  two  streams  of  waves 
may  come  together  and  destroy  each  other,  that  is,  produce 
darkness. 


GILBERT  LIGHT  EXPERIMENTS  93 


Colors  by  Interfer- 
ence. If  a  beam  of 
sunlight  is  allowed  to 
fall  on  a  soap  film  held 
in  a  vertical  position 
on  the  end  of  a  lamp 
chimney  (Fig.  144),  it 
is  found  that  the  soap 
film  when  viewed  by 
reflected    light  is 


Crossed  by  horizontal  Fig.  144.    Colors  in  a  soap  film 

colored  bands.  These 

colors  are  formed  by  interference  as  follows :  The  soap  film  has 
two  surfaces  with  water  between,  and  when  it  stands  on  edge 
the  water  runs  toward  the  bottom  and  the  film  becomes  a  narrow 
prism.  Now  the  light  is  reflected  partly  from  the  front  film  and 
partly  from  the  back  film,  and  where  the  films  are  1-4,  3-4,  5-4 
waves  of  red  light  apart,  the  red  waves  from  the  rear  are  1-2, 
1  1-2,  2  1-2  waves  behind  the  red  waves  from  the  front  when  they 
enter  your  eye.  These  two  sets  of  waves,  then,  interfere  and  de- 
stroy each  other,  and  all  that  your  eye  sees  is  blue.    Similarly  a 


little  above  and  below 
these  points  the  blue 
waves  destroy  each 
other,  and  you  see  red 
light. 


FUN   WITH  SUN- 
LIGHT 


Fig.  145.    You  produce  a  beautiful  spectrum  by 
means  of  your  prism 


Experiment  No.  84. 

The  prism  spectrum. 
Allow  a  beam  of  sun- 
light to  pass  through 


94 


GILBERT  BOY  ENGINEERING 


Fig.  146. 


You  produce  a  beautiful  spectrum  by 
reflection  under  water 


the  slit  in  your  dark- 
ened room  and  fall  on 
the  prism  supported  be- 
tween blocks  as  shown 
in  Fig.  145.  Cut  a  piece 
of  cardboard  of  the  ex- 
act size  of  one  face  of 
the  prism  and  put  it  on 
the  upper  face.  Do  you 
find  a  beautiful  spec- 
trum on  the  wall  or  ceil- 
ing? Do  you  find  that 
the  violet  end  is  nearest 
the  base  of  the  prism  and  the  red  end  nearest  the  angle,  that  is, 
is  the  violet  end  the  most  bent?  Turn  the  prism  over.  Do  you 
get  a  spectrum  on  the  floor? 

Get  the  spectrum  on  the  wall  or  ceiling  again  and  rock  the 
prism  quickly.  Is  the  center  of  the  spectrum  white?  This  proves 
that  white  light  is  made  up  of  all  spectrum  colors  because  they 
mix  at  the  center. 

Experiment  No.  85.  Spectrum  by  reflection.  Place  a  mirror 
in  a  slanting  position  under  water  and  arrange  it  so  that  the 
beam  of  sunlight  falls  on  the  mirror  (Fig.  146).  Do  you  find  a 
beautiful  spectrum  on  the  wall  above  the  slit?  Stir  the  water. 
Do  the  colors  mix  and  produce  white  light? 

Experiment  No.  86.  Colors  by  interference.  Make  soap  suds 
as  you  would  for  blowing  soap  bubbles.  Put  the  suds  in  a  saucer. 
Dip  the  end  of  a  lamp  chimney  in  the  suds  and  support  the  chim- 
ney on  its  side  in  sunlight  (Fig.  144).  Look  at  the  film  by  reflected 
light.  Do  you  find  that  the  film  at  the  top  is  crossed  by  beautiful 
horizontal  colored  bands?  These  colors  are  produced  by  inter- 
ference. The  colors  in  a  soap  bubble  and  in  a  film  of  oil  on  water 
are  produced  by  interference. 


GILBERT  LIGHT  EXPERIMENTS  95 


WHY  OBJECTS  ARE  COLORED 

An  object  has  a  certain  color  because  something  in  the  object 
absorbs  all  other  colors.  For  example,  a  blue  dress  is  blue  be- 
cause the  dye  in  the  dress  absorbs  all  the  other  colors  of  the  spec- 
trum. Also  a  red  dress  is  red  because  the  dye  absorbs  the  colors 
in  the  blue  end  of  the  spectrum,  and  so  on. 

An  object  is  white  when  all  of  the  colors  of  the  spectrum  are 
partly  absorbed  and  all  are  reflected. 

An  object  is  black  when  all  the  colors  are  completely  ab- 
sorbed and  none  reflected. 

Experiment  No.  87.  Changing  colors.  Darken  your  room  and 
allow  sunlight  to  enter  through  a  slit  smaller  than  your  colored- 
glass  plates.  Hold  the  red  glass  over  the  slit  and  hold  colored 
objects  in  the  red  light.  Are  red  objects  red,  but  all  other  colored 
objects  dark  or  black?  They  are  dark  or  black  because  the  dye 
in  them  absorbs  the  red  light.  Repeat  with  the  blue  glass. 
Are  the  results  similar? 

Note.  The  blue  glass  lets  through  a  little  red,  yellow,  and 
green,  as  you  will  now  show. 

Experiment  No.  88.  Changed  spectrum.  Get  the  spectrum 
with  the  prism  and  then  put  the  red  glass  against  the  prism. 
Does  the  red  glass  absorb  all  colors  except  red  ?  Repeat  with  the 
blue  glass.  Does  it  absorb  nearly  all  colors  except  blue,  but 
does  it  let  through  a  small  amount  of  the  other  colors? 

Experiment  No.  89.  Changing  colors  in  spectrum.  Get  the 
spectrum  with  the  prism  and  hold  colored  objects  in  the  differ- 
ent colors.  Do  they  change  colors  according  to  the  part  of 
the  spectrum  they  are  in?  They  are  black  in  the  part  of  the 
spectrum  which  they  absorb  completely. 

FUN  BY  DAY  OR  NIGHT 
Experiment  No.  90.    A  colored  strip.    Cut  a  strip  of  white 
paper  about  1-16  inch  wide  and  2  inches  long  and  pin  it  to  a 


96 


GILBERT  BOY  ENGINEERING 


Fig.  147.   The  white  paper  is  colored 


black  object.  Put 
it  in  a  good  light 
and  look  at  it 
through  the  prism 
(Fig.  147).  Do  you 
find  a  spectrum  in- 
stead of  the  white 
paper? 

Do  you  find  also 
that  the  spectrum  is 
reversed,  that  is, 
that  the  red  is  nearest  the  base  of  the  prism  and  the  violet 
nearest  the  angle?  This  is  so  because  your  eye  sees  an  object 
in  the  direction  the  light  enters  it  from  the  object.  The  red 
is  least  bent  but  appears  to  be  most  bent,  and  the  violet  the 
reverse. 

Experiment  No.  91.  Combining  spectra.  Cut  a  strip  of 
white  paper  1  inch  wide  and  2  inches  long  and  look  at  it  through 
the  prism  (Fig.  148).  Do  the  edges  appear  colored,  but  is  the 
center  white?  The  center  is  white  because  the  spectra  formed 
by  the  edges  overlap  at  the  center  and  this  combination  of  all  the 
colors  of  the  spectrum  produces  white  light. 

Experiment 
No.  92.  Colored  can- 
dle, flame.  Look  at 
the  flame  of  a  candle 
through  a  prism. 
Is  it  beautifully  col- 
ored,  but  does 
the  center  tend  to 
be  white  and  are  the 
colors     reversed  as 

above  ?  j?ig.  148.   The  paper  is  colored  only  at  the  edge 


GILBERT  LIGHT  EXPERIMENTS  97 


COMPLEMENTARY  COLORS 

Complementary  colors  are  those  which,  when  combined,  pro- 
duce white  light.  If  any  colors  are  taken  out  of  the  spectrum, 
the  remaining  colors  are  complementary  to  those  taken  out, 
because  together  they  produce  white  light. 

MIXING  PAINTS 

A  paint  which  absorbs  the  colors  in  the  blue  end  of  the  spec- 
trum is  red  in  color  and  a  paint  which  absorbs  the  colors  in 
the  red  end  of  the  spectrum  is  blue  in  color.  If,  now,  these 
paints  are  mixed,  they  do  not  produce  white  paint  but  black 
paint  because  together  they  absorb  all  the  colors. 

FUN  WITH  SUNLIGHT 
Experiment  No.  93.  Colored  glasses.  Stand  the  red  and 
blue  glasses  side  by  side  on  a  piece  of  white  paper  in  sunlight. 
The  red  absorbs  the  blue  end  of  the  spectrum  and  lets  through 
red  light.  The  blue  absorbs  the  red  end  of  the  spectrum  and 
lets  through  blue  light.  Now  place  one  behind  the  other.  Do 
they  absorb  all  the  light  and  is  the  shadow  black? 

THE  RAINBOW 

The  rainbow  (Fig. 
149)  is  formed  by  the 
internal  reflection  and 
dispersion  of  sunlight 
by  falling  drops  of 
water.  You  see  itwhen 
the  sun  is  . behind  you 
and  not  over  42°  above 
the  horizon.  The  first 
or  primary  rainbow  is 
formed  by  two  refrac- 
K  —  7 


Fig.  149  (1).  The  "why"  of  the  rainbow 
From  the  Ontario  High  School  Physics,  by  permis- 
sion of  the  publishers 


98 


GILBERT  BOY  ENGINEERING 


Fig.  149  (2) 
From  the  Ontario  High  School  Physics,  by  permis- 
sion of  the  publishers 


tions  at  A  and  C  and 
one  internal  reflection 
at  B  (2)  ;  it  is  violet 
below  and  red  above 
and  the  angle  at  which 
the  light  enters  your 
eye  is  about  41°  to  the 
direction  of  the  sun- 
light.   The  secondary  rainbow  is  formed  by  two  refractions  A 
and  D  and  two  internal  reflections  B  and  C  (3)  ;  it  is  red  below 
and  violet  above  and  the  angle  of  the  light  is  about  52°. 

Experiment  No.  94.  An  artificial  rainbow.  Place  a  glass  full 
of  water  (Fig.  150)  on  a  table  in  sunlight  and  projecting  beyond 
the  edge.  Do  you  get  two  or  more  beautiful  rainbows  on  the  floor? 
Stand  the  glass  on  a  mirror.  Do  you  get  two  beautiful  rainbows 
on  the  ceiling?   These  bows,  however,  are  not  reversed,  This 


Fig.  150.    You  make  an  artificial  rainbow 


GILBERT  LIGHT  EXPERIMENTS  99 


Fig.  151.    You  see  a  devil 


experiment  will  show 
best  in  your  darkened 
room. 

FUN  AT  NIGHT 
Experiment  No.  95. 

A  changing  devil.  Cut 
a  little  devil  out  of 
cardboard  and  arrange 
as  shown  in  Fig.  151. 
Hold  the  red  glass  in 

front  of  the  candle  at  the  right.  Is  the  devil  at  the  right  red  and 
is  the  devil  at  the  left  very  dim  but  of  the  complementary  color, 
green?  Use  blue  glass.  Is  one  devil  blue  and  the  other  very 
dim  but  of  the  complementary  color,  orange? 

Experiment  No.  96.  A  tri- 
colored  star.  Fold  a  piece  of 
cardboard.  Cut  a  four-pointed 
star  in  one  half.  Fold  the 
points  back,  make  a  tracing  on 
the  other  half,  and  cut  out  a 
star  very  carefully  with  the 
points  exactly  between  those 
of  the  first.  Arrange  as  shown 
in  Fig.  152  and  hold  the  red 
glass  in  front  of  one  can- 
dle. Do  you  get  an  eight- 
pointed  star  with  the  points 
alternately  red  and  green  and 
with  a  white  or  pink  eight- 
pointed  star  inside?  Repeat 
with  the  blue  glass.    Are  the 

Fig.  152.    You  see  an  eight-pointed  star      points  blue  and  Orange? 


100  GILBERT  BOY  ENGINEERING 


Fig.  153.   Trench  faces 


Experiment  No.  97.  A  ghost  party.  Mix  a  half  teaspoonfnl 
of  salt  in  three  or  four  teaspoonfuls  of  alcohol  in  a  saucer,  stand 
the  saucer  on  a  cup  on  the  table  (to  prevent  burning  the  table), 


GILBERT  LIGHT  EXPERIMENTS  101 


seat  the  party  around  the  table  in  the  dark,  light  the  alcohol,  and 
look  at  your  neighbors'  faces  and  at  your  own  in  a  mirror.  Do 
you  all  look  like  ghosts  ?  You  do,  because  the  salt  in  the  flame 
gives  only  yellow  light,  and  since  your  rosy  cheeks  and  rosy  lips 
absorb  this  color  they  appear  black. 

TRENCH  FACES 

Our  boys  at  the  front  painted  their  faces  black  (Fig.  153) 
before  they  started  out  on  night  raids,  because  the  black  paint 
absorbed  the  light  and  prevented  their  faces  from  being  seen. 


Fig.  154.  Spectroscope 
Courtesy  of  the  Scientific  American 

THE  SPECTROSCOPE 

When  substances  are  vaporized  in  a  flame  and  the  flame  is 
viewed  through  a  spectroscope  (Fig.  154)  the  spectrum  seen  is 
crossed  by  bright  lines.  Each  substance  has  its  own  particular 
lines,  and  when  we  know  these  lines  we  can  tell  what  substances 
are  in  the  flame.  This  is  the  basis  of  spectrum  analysis.  In  the 
spectroscope  shown  here  the  light  passes  through  a  narrow  slit, 
through  tube  A,  through  four  prisms,  and  into  the  telescope  B 
in  which  the  enlarged  spectrum  is  seen. 


102  GILBERT  BOY  ENGINEERING 


WHAT  IS  IN  THE  SUN  AND  STARS? 

When  the  light  from  the  stars  is  viewed  in  the  spectroscope, 
the  spectrum  is  crossed  by  dark  lines  exactly  corresponding  to 
the  bright  lines  mentioned  above.  These  are  called  the  Fraun- 
hofer  lines,  after  their  discoverer.  If,  in  the  spectrum  of  light 
from  the  sun,  for  example,  we  see  dark  lines  exactly  corre- 
sponding to  the  bright  lines  produced  by  iron  in  the  spectrum 
on  the  earth,  we  know  that  there  is  iron  in  the  sun,  and  so  on. 


3 

Fig.  155.    A  lighthouse  lens 
From  Aldons*  Elementary  Course  of  Physics,  published  by  The  Macmillan  Go. 

LIGHTHOUSE  LENSES 

Lighthouse  lenses  have  at  the  center  a  comparatively  thin 
lens  and  around  this  prismatic  sections  with  greater  and  greater 
angle  toward  the  edge,  (1)  Fig.  155.  Panels  (2)  made  up  in 
this  way  are  placed  completely  around  the  light  F  (3).  This 
gives  a  large,  short  focus  lens  which  does  not  absorb  as  much 
light  as  a  solid  thick  lens  would  absorb. 

LENSES 

Lenses  are  of  two  kinds,  converging  and  diverging. 
Converging  lenses  are  thicker  at  the  middle  than  at  the  edges, 
and  we  may  think  of  them  as  made  up  of  sections  of  prisms, 


GILBERT  LIGHT  EXPERIMENTS  103 


Fig.  156  (1).  A  converging  Fig.  156  (2).    A  diverging 

lens  as  sections  of  prisms  lens  as  sections  of  prisms 

From  Lynde's  Physics  of  the  Household,  published  by  The  Macmillan  Co. 


Fig.  156  (1),  the  angles  of  the  prisms  being  greater  the  nearer 
they  approach  the  edges.  These  lenses  converge  parallel  rays 
to  a  point  F,  called  the  focus. 

(Diverging  lenses  are  thinner  at  the  middle  than  at  the  edges, 
and  we  may  think  of  them  as  made  up  of  sections  of  prisms, 
Fig.  156  (.2),  with  their  thin  edges  toward  the  center.  These 
lenses  diverge  parallel  rays  and  make  them  appear  to  come 
from  a  point  F,  called  an  unreal  or  virtual  focus. 


FUN  WITH  SUNLIGHT 


Experiment  No.  98. 

pass  through  the  slit  in 
ing  lens  in  the  beam 
(Fig.  157)  and  make  a 
dust.  Do  you  see  that 
the  light  comes  to  a 
point  and  diverges  af- 
terward ? 

Repeat  with  the 
other  converging  lens. 
Is  the  light  again 
brought  to  a  point  but 
at  a  different  distance 
from  the  lens? 


Converging  lenses.  Allow  sunlight  to 
your  darkened  room,  hold  a  converg- 


Fig.  157.   You  see  a  brilliant  focus  in  dusty  ail 


104 


GILBERT  BOY  ENGINEERING 


Experiment  No.  99. 

Diverging  lens.  Re- 
peat this  experiment 
with  your  diverging 
lens.  Is  the  light  di- 
verged or  spread? 

Experiment 
No.  100.  Focal  lengths. 
Remove  your  shutter, 

Fig.  158.   You  measure  the  focal  length  focus  the  light  with  a 

converging  lens,  hold 
a  piece  of  paper  at  the  point  where  you  get  the  smallest  and 
brightest  image  of  the  sun  (Fig.  158)  and  measure  the  distance 
from  the  lens  to  the  paper.  The  point  is  the  focus  and  the  dis- 
tance is  the  focal  length  of  the  lens. 

Repeat  with  the  other  converging  lens. 

£)o  you  find  the  focal  lengths  of  the  lenses  to  be  4  inches 
and  8  inches  respectively? 

Experiment  No.  101.  Focal  length  of  diverging  lens.  Punch 
two  nail  holes  exactly  1  inch  apart  in  a  piece  of  paper,  put  this 
in  front  of  the  diverging  lens,  and  measure  the  distance  at  which 
the  spots  of  sunlight  appear  2  inches  apart  on  a  paper  behind 
the  lens.  This  is  the 
virtual  focal  length. 
Is  it  4  inches? 

Experiment 
No.  102.  Is  it  hot? 
Put  your  hand  at  the 
focus  of  each  converg- 
ing lens  in  turn  (Fig. 
159).  Is  the  sunlight 
hot?  It  is,  because  all 

the    light    and    heat  Fig.  159.  The  focus  is  hot 


GILBERT  LIGHT  EXPERIMENTS  105 


which  falls  on  the  lens 
is  concentrated  at  the 
focus. 

Repeat  with  the  di- 
verging lens.  Is  there 
no  heat? 

Experiment 
No.  103.  To  light  a 
match  with  sunlight. 
When  the  sun  is  hot 
at     mid-day     put     a  Fig*  160,    ^ou  ^S^t  a  match  with  sunlight 

match  on  a  piece  of 

paper  and  focus  sunlight  on  it  with  the  short  focus  lens  (Fig.  160). 
Does  it  light?  Why? 

Experiment  No.  104.  Magic  cannon.  Repeat  Experiment 
No.  59,  but  light  the  match  by  means  of  the  short  focus  lens 
(Fig.  161). 

THE  "WHY"  OF  IT 

When  the  parallel  waves  from  the  sun  fall  on  a  converging 
lens,  which  is  thicker  at  the  middle  than  at  the  edges  (Fig.  162), 

the  portions  of  the 
waves  that  go  through 
the  thick  part  are 
slowed  up  more  than 
the  portions  which  go 
through  the  thinner 
parts,  and  as  a  result 
the  waves  are  so 
curved  in  that  they 
converge  at  the  focus 
and  diverge  afterward. 
Fig.  161.    You  light  the  match  in  the  bottle  The  waves  are  shown 


106  GILBERT  BOY  ENGINEERING 


i  2 

Fig.  162.    Parallel  wave3  and  rays  are  converged 


in  1  and  the  rays  in  2.  This  explains  why  these  lenses  con- 
verge the  light. 

When  parallel  waves  fall  on  a  diverging  lens,  which  is  thinner 
at  the  center  than  at  the  edges,  the  portions  which  go  through 
the  center  are  less  delayed  than  the  portions  which  go  through 
the  edges  and  the  waves  are  so  curved  out  that  they  diverge 
after  passing  through  the  lens.  The  waves  are  shown  in  1, 
Fig.  163,  and  the  rays  in  2.  This  explains  why  these  lenses 
diverge  the  light. 

If  the  light  comes  from  an  object  near  a  converging  lens 
the  waves  are  curved  when  they  reach  it,  and  one  of  three  things 
may  happen. 

If  the  object  is  at  a  distance  from  the  lens  greater  than  the 
focal  length  (1,  Fig.  164),  the  curvature  of  the  waves  is  reversed 
and  the  light  is  brought  to  a  point  on  the  other  side  of  the  lens 


Fig.  163,    Parallel  waves  and  rays  are  diverged 


GILBERT  LIGHT  EXPERIMENTS  107 


at  a  distance  greater 
than  the  focal  length. 

If  the  light  is  at 
the  focus  (2,  Fig.  164), 
the  curvature  of  the 
waves  is  so  altered 
that  they  are  parallel 
after  they  pass 
through  the  lens. 

If  the  light  is 
nearer  to  the  lens  than 
the  focus  (3,  Fig.  164), 
the  curvature  of  the 
waves  is  altered  by 
the  lens,  but  they  still 
diverge  and  will  never 
converge. 


Fig.  164.    Light  and  a  converging  lens 


FUN  BY  DAY  OR  NIGHT 
Experiment  No.  105.  Images.  Arrange  a  candle,  4-inch  con- 
verging lens,  and  screen  as  in  Fig.  165.   Place  the  lighted  candle 

3  feet  from  the  lens 
and  move  the  screen 
until  you  get  an  image. 
Is  it  inverted  and 
small  ?  Repeat  with 
candle  at  2  feet  and  1 
foot.  Is  the  image 
larger  each  time? 

Place    candle  at 
twice  the  focal  length, 
that  is,  8  inches.  Are 
Fig.  165.   You  see  a  picture  of  the  candle  image  and  candle  the 


108 


GILBERT  BOY  ENGINEERING 


same  size?  Place  can- 
dle at  6  inches.  Is  the 
image  larger?  Place 
candle  at  5  inches.  Is 
the  image  larger  still? 
Place  candle  at  the  fo- 
cus. Is  the  image  very- 
large?  Place  candle  at 
3  inches  and  2  inches, 
that  is,  closer  than  focus. 

Pig.  166.  You  see  a  picture  of  your  hand  A™  no  images  formed  ? 

Repeat    with  the 

converging  lens  of  8-inch  focus.  Place  candle  at  distance  of 
4  feet,  3  feet,  2  feet,  16  inches  or  twice  the  focal  length,  15  inches, 
12  inches,  8  inches,  and  6  inches.    Are  the  results  similar? 

Is  the  image  smaller  than  the  candle  when  the  candle  is  at  a 
greater  distance  from  the  lens  than  twice  the  focal  length?  Is  it 
larger  when  the  candle  is  at  a  distance  less  than  twice  the  focal 
length  and  greater  than  the  focal  length? 

Experiment  No.  106.  Picture  shows.  With  the  candle,  con- 
verging lens,  and  screen,  as  in  Fig.  166,  get  the  image  of  the 
candle  on  the  screen,  then  hold  your  hand  behind  the  candle 
and  close  to  it.  Do  you  get  an  inverted  picture  of  your  hand  in 
natural  colors? 

Hold  a  black  and  white  drawing  upside  down  and  close  to 
the  candle.    Do  you  get  a  picture  right  side  up? 

Repeat  with  colored  drawings,  colored  flowers,  and  so  on. 
Do  you  get  colored  pictures? 

Repeat  with  all  kinds  of  things  and  use  four  or  five  candles 
to  get  more  light. 

Experiment  No.  107.  A  picture  of  out-of-doors.  In  the  day- 
time, go  to  the  side  of  the  room  away  from  the  window  and  get 
a  picture  of  distant  objects  on  the  screen  (Fig.  167).    Do  you 


GILBERT  LIGHT  EXPERIMENTS  109 


find  a  beautiful  invert- 
ed picture  in  natural 
colors  of  everything 
out-of-doors  ? 

Measure  the  dis- 
tance from  lens  to 
screen.  This  is  again 
the  focal  length  of  the 
lens.  At  night  get  a 
picture  of  a  distant 
light  and  measure  the  Fig  167  You  see  a  picture  of  thingS  out-of-doors 
focal  length. 

Experiment  No.  108.  The  lenses  and  your  eyes.  Hold  the 
converging  lenses  in  turn  at  arm's  length  and  look  at  distant 
objects.    Is  the  image  small  and  inverted? 

Hold  them  about  one  foot  from  your  eye  and  look  at  your 
finger  held  closer  to  the  lens  than  its  focal  length.  Is  the  image 
large  and  right  side  up? 

Repeat  with  the  diverging  lens.  Is  the  image  always  right 
side  up  and  small? 

HOW  THE  IMAGES  ARE  FORMED 

In  Fig.  168  (1)  the  object  OB  is  at  a  greater  distance  than  the 
focal  length.  All  the  rays  which  fall  on  the  lens  from  any  point 
B  meet  at  the  point  M  and,  therefore,  the  image  of  B  is  at  M. 
We  cannot  trace  all  the  rays,  but  it  is  necessary  to  trace  only  two. 
The  two  most  easily  traced  are  the  parallel  ray  BR  and  the  ray 
BP  which  goes  through  the  center  of  the  lens.  Ray  BR  goes 
through  the  focus  F  after  it  goes  through  the  lens ;  ray  BP  goes 
straight  ahead,  or  nearly  so,  because  the  two  sides  of  the  lens 
are  nearly  parallel  at  the  center. 

The  rays  from  all  other  points  between  B  and  O  meet  at  points 
between  M  and  I  and,  therefore,  MI  is  the  inverted  image  of  BO. 


110 


GILBERT  BOY  ENGINEERING 


(2)  (3) 
Fig.  163.    How  images  are  formed 
From  Lynde's  Physics  of  the  Household,  published  by  The  Macmillan  Co. 


In  (2),  BO  is  inside  the  focus;  therefore  BR  and  BP  diverge 
after  they  pass  through  the  lens  and  do  not  form  an  image.  Your 
eye,  however,  makes  an  image  because  it  sees  the  rays  as  though 
they  came  from  MI.  This  explains  why  you  see  anything  inside 
the  focal  length  as  enlarged  and  right  side  up. 

In  (3),  BO  is  outside  the  virtual  focus  of  the  diverging  lens. 
BR  and  BP  diverge  after  they  pass  through  the  lens  and  your 
eye  sees  the  image  MI.  This  explains  why  diverging  lenses 
always  give  images  small  and  right  side  up. 

POWER  OF  A  LENS 

Spectacles  are  lenses,  and  opticians  measure  the  power  of  the 
spectacle  lenses  as  follows :  If  the  lens  has  a  focal  length  of 
1  meter  it  is  said  to  have  a  power  of  1  diopter ;  if  it  has  a  focal 
length  of  1-2,  1-3,  or  1-10  meter  it  is  said  to  have  a  power  of  2,  3, 
or  10  diopters ;  and  so  on.  That  is,  the  shorter  the  focal  length 
the  greater  the  power. 


GILBERT  LIGHT  EXPERIMENTS 


111 


A  meter  is  100  centimeters  long".  You  will  find  on  most  or- 
dinary rulers  30  divisions  on  the  side  opposite  the  inch  divisions ; 


Fig.  169.    Conjugate  foci 


each  of  these  divisions  is  1  centimeter,  and  100  of  these  make 
a  meter. 

Experiment  No.  109.  Power  of  your  lenses.  Measure  in 
centimeters  the  focal  length  of  the  8-inch  lens.    Do  you  find 

100 

it  to  be  20  cms.  ?  Is  the  power  of  the  lens  then  =  5  diopters  ? 

20 

Repeat  with  the  4-inch  lens.   Is  its  focal  length  10  cms.  and 
100 

its  power  ==  10  diopters  ? 

10 

Experiment  No.  110.  Power  of  spectacles.  Measure  in  centi- 
meters the  focal  length  of  your  father's  or  mother's  spectacles 
and  calculate  their  power  in  diopters. 

Experiment  No.  111.  Conjugate  foci.  Get  the  image  of  a 
candle  as  in  Fig.  169,  mark  the  position  of  the  screen  and  the 
candle,  and  then  exchange  them.  Do  you  again  find  an  image, 
but  of  different  size? 

Repeat  at  different  distances. 

Two  points  so  situated  with  respect  to  a  converging  lens  that 
an  object  at  either  forms  an  image  at  the  other  are  called  conjugate 
foci.  There  are  an  infinite  number  of  pairs  of  such  points  for 
each  converging  lens. 


112 


GILBERT  BOY  ENGINEERING 


RELATION  BETWEEN  OBJECT  AND  IMAGE 

If  DQ  is  the  distance  of  an  object  from  a  lens  and  Di  is  the 

111 

distance  of  its  image  from  the  lens,  then  —  +  —  —  — ,  where  F  is 

D0    D,  F 

the  focal  length  of  the  lens.  This  is  one  relation  between  the 
object  and  its  image. 

The  magnification  of  an  image  is  the  number  of  times  it  is 
larger  or  smaller  than  the  object,  and  you  can  always  find  it  by 
dividing  Dj  by  DQ;  that  is,  the  magnification  =  Dj-f-  DQ. 

Experiment  No.  112.  Where  is  the  image?  Arrange  the  4- 
inch  lens  with  the  candle  6  inches  from  it.  Calculate  where  the 
image  will  be  as  follows : 

1      11      1      1  _  1      1  _  1      1_3       2  _  1 

D~Q  F OI  6     D"i~40rD"i~4  — 6~12"~12  ~12 

.*.       is  12.  The  image  will  be  12  inches  from  the  lens.  Try  it. 
Now  calculate  and  try  where  the  image  will  be  if  the  object 
is  5  inches,  7  inches,  8  inches,  12  inches,  20  inches  from  the  lens, 
and  so  on. 

Repeat  with  the  8-inch  lens,  using  DQ  greater  than  8  inches. 

Experiment  No.  113.  How  big  will  the  image  be?  Arrange 
the  candle  6  inches  from  the  4-inch  lens  and  the  image  will  be 
at  12  inches,  as  you  found  above. 

Now,  since  magnification  =  Dj  DQ,  it  is  12  -r-  6  =  2,  and  the 
image  will  be  2  times  as  large  as  the  object.  Measure  the  height 
of  the  flame  and  of  its  image.  Is  the  image  2  times  as  high  as 
the  flame?  Try  other  distances  and  then  the  other  lens. 

MAGIC 

Experiment  No.  114.  Cylindrical  lens.  Look  at  your  finger 
through  a  tumbler  of  water.  Does  the  tumbler  of  water  act  as 
a  cylindrical  lens  and  is  your  finger  broad? 


GILBERT  LIGHT  EXPERIMENTS  1 1 3 


Experiment  No.  115.  Treble  your  money.  Put  a  quarter  in  a 
tumbler  half  full  of  water,  put  a  saucer  over  the  tumbler,  and  in- 
vert both.  Do  you  see  a  half  dollar  on  the  saucer  and  a  quarter 
higher  up?  Why? 

Experiment  No.  116.  Heat  through  ice.  Place  the  concave 
mirror  upside  down  on  a  sheet  of  clear  ice  inch  thick  and  let 
it  melt  into  the  ice.  Do  you  get  an  ice  lens  ?  At  noon,  when  the 
sun  is  hot,  hold  your  hand  at  the  focus  of  this  lens.    Is  it  hot? 

Experiment  No.  117.  A  spectrum  from  ice.  Take  a  clear  piece 
of  ice,  shave  it  to  the  shape  of  a  prism,  and  hold  it  in  sunlight. 
Do  you  get  a  beautiful  spectrum? 


OPTICAL  INSTRUMENTS 


FUN  BY  DAY  OR  NIGHT 

A  Magnifying  Glass  is  simply  a  converging  lens  (Fig.  170) 
with  the  object  PQ  closer  than  the  focus.  The  eye  receives  rays 
which  are  still  diverging  and 
sees  the  image  pq  enlarged. 
You  have  illustrated  this 
above. 

The  Astronomical  Tele- 
scope (Fig.  171)  consists  of 
two  converging  lenses,  or 
systems  of  lenses,  connected 
by  a  long  tube.  The  lens 
nearest  the  object  is  called 
the  objective,  and  the  lens  nearest  the  eye,  the  eyepiece. 

The  objective  (Fig.  172)  forms  a  real  inverted  image  im  of 
the  object  BO  inside  the  focus  of  the  eyepiece.  The  eyepiece 
magnifies  this,  just  as  a  magnifying  glass  does,  and  the  eye  sees 
the  enlarged  image  IM. 

When  the  telescope  is  focused  on  a  distant  object:  the  dis- 

K  — 8 


Fig.  170.    A  magnifying  glass 
From  Lynde's  Physics  of  the  Household, 
published  by  The  Macmillan  Co. 


114  GILBERT  BOY  ENGINEERING 


Fig.  171.   Astronomical  telescope  at  Lick  Observatory 
Courtesy  of  the  Scientific  American 


GILBERT  LIGHT  EXPERIMENTS 


115 


Fig.  172.    How  you  see  the  image 
From  Lynde's  Physics  of  the  Household,  published  by  The  MacmilJan  Go. 


tance  between  the  lenses  is  equal  to  the  sum  of  their  focal 
lengths ;  and  the  magnification  is  equal  to  the  focal  length  of  the 
objective  divided  by  the  focal  length  of  the  eyepiece. 

Terrestrial  telescopes  have,  between  the  objective  and  eye- 
piece, other  lenses  which  turn  the  image  right  side  up. 

Experiment  No.  118.  An  astronomical  telescope.  Arrange 
the  converging  lenses  on  a  piece  of  board  (Fig.  173)  and  focus 
on  a  distant  object. 

Measure  the  distance  between  the  lenses.  Is  it  equal  to  the 
sum  of  their  focal  lengths,  that  is,  8  -f-  4  =  12  inches  ? 

Look  at  a  distant  object  through  the  telescope  with  one  eye 
and  outside  the  telescope  with  the  other  eye.  Is  the  magnifi- 
cation equal  to  focal 
length  of  objective  -f- 
focal  length  of  eye- 
piece, that  is,  8  -T-  4 
=  2  times  ? 

Hold  a  piece  of 
paper  at  the  focus  of 
the  objective.  Do  you 
get  an  image? 

Experiment 
No.  119.    To  make  a 
telescope.     Place  8- 
Fig.  173.    You  illustrate  the  telescope  inch  lens  in  ring  hold- 


116 


GILBERT  BOY  ENGINEERING 


Fig.  174.    You  make  a  telescope 

into  the  first  and  your  telescope  is 
made  (Fig.  174).  Focus  it  on  a 
distant  object. 

The  Compound  Microscope 
(Fig.  175)  is  the  same  in  principle 
as  the  astronomical  telescope,  but 
the  objective  has  very  great  power, 
that  is,  it  has  a  very  short  focal 
length.  The  objective  forms  a 
real  image,  im,  Fig.  176,  of  BQ, 
and  the  eyepiece  forms  the  en- 
larged image  IM  of  im. 

The  Opera  Glass  (Fig.  177) 
has  a  converging  lens  C  for  objec- 
tive and  a  diverging  lens  c  for 
eyepiece.  The  objective  would 
form  an  inverted  image  ab  of  AB, 
but  the  eyepiece  diverges  the  light 
and  the  eye  sees  the  erect  image 
A'B'.  The  ordinary  opera  glass 
consists  of  two  such  instruments; 


er  and  wind  dark 
wrapping  paper 
around  the  holder  to 
make  a  tube  10  inches 
long.  Place  4-inch 
lens  in  the  other 
ring  holder  and  wind 
wrappingpaper  around 
the  holder  to  make 
a  tube  6  inches  long. 
Slip  the  second  tube 


Fig.  175.    A  compound  microscope 


GILBERT  LIGHT  EXPERIMENTS  1 1  7 


Fig.  176.    Illustrating  how  images  are  formed  in  the  microscope 
From  Lynde's  Physics  of  the  Household,  published  by  The  Macmillan  Go. 


they  are  shorter  than  the  ordinary  telescope  and,  therefore,  more 
convenient. 


Fig.  177.    How  you  see  things  in  an  opera  glass 
From  Lynde's  Physics  of  the  Household,  published  by  The  Macmillan  Co. 

Experiment  No.  120.  An  opera  glass.  Arrange  the  lenses 
on  a  piece  of  board  as  in  Fig.  178.  Focus  on  an  object.  Is 
the  image  erect  and 
are  the  lenses  closer 
together  than  in  the 
telescope  ? 

Experiment 
No.  121.  To  make  an 
opera  glass.  Place  8- 
inch  lens  in  ring  holder 
and  wind  around  it  a 
tube  of  wrappingpaper 

3  inches  long.    Place  Fig.  178.  You  illustrate  tne  opera  glasa 


118  GILBERT  BOY  ENGINEERING 


I 

Fig.  179.  Binoculars 

Courtesy  of  the  Scientific  American 


the  diverging  lens  in  the  other  ring  holder  and  wind  a  tube 
2  inches  long.  Insert  the  second  tube  in  the  first  and  your  opera 
glass  is  made.  Focus  it  on  a  distant  object. 

The  Prism  Binoculars  (Fig.  179)  are  made  with  lenses  sim- 
ilar to  those  in  an  astronomical  telescope,  but  the  light  is  re- 
flected four  times  by  means  of  glass  prisms.  This  reflection 
makes  the  image  erect  and  shortens  the  length  of  the  tube. 


The  Projecting 
Lantern  (Fig.  180) 
consists  of  a  light- 
proof  box,  a  source 
of  bright  light,  a  con- 
densing lens,  a  lantern 
slide,  and  a  projecting 
lens.  The  bright  light, 


Fig.  180.    A  projecting  lantern  produced   by  eleCtriC- 

From  Lynde's  Physics  of  the  Household,  published        m  t 

by  The  Macmiiian  Co.  lty,  acetylene,  or,  as 


GILBERT  LIGHT  EXPERIMENTS 


119 


Fig.  181.    You  make  a  postcard  lantern 


here,  by  a  limelight, 
is  converged  on  the 
lantern  slide  by  the 
condensing  lens  and 
an  image  of  the  in- 
verted slide  is  thrown 
on  the  screen  by  the 
projecting  lens. 

The  Postcard  Lan- 
tern consists  of  a 
light-proof  box,  two 
electric  lights  which 
throw  light  on  the  postcard  but  not  directly  on  the  lens,  a 
postcard  slide,  and  a  converging  lens  which  throws  an  image 
of  the  postcard  on  the  screen. 

Experiment  No.  122.  Magic-lantern  shows.  Place  4-inch 
lens  in  ring  holder  in  a  hole  in  a  large  piece  of  cardboard,  place 
a  black  book  6  inches  from  lens  and  a  white  screen  12  inches 
from  lens  on  the  other  side,  light  the  candles,  and  hold  small 
objects  against  the  book.  Are  their  images  thrown  on  the 
screen  in  natural  colors  and  magnified  twice? 

Experiment 
No.  123.  To  make  a 
postcard  lantern.  You 
can  have  lots  of  fun 
with  a  lantern  made  as 
follows : 

Get  a  cardboard  or 
wooden  box  (Fig.  181) 
about  8"  X  6"  X  6", 
put  the  8-inch  lens  in 
ring  holder  and  in  a 

Fig.  182,   You  hold  a  magic-lantern  show  wrapping  paper  tube 


120 


GILBERT  BOY  ENGINEERING 


A, 


a 


4  inches  long ;  put  the 
tube  into  a  hole  in 
one  side  of  the  box 
and  paint  the  opposite 
side  of  the  box  black. 
Place  an  electric  light 
or  oil  lamp  on  each 


Fig.  183.    A  camera 
From  Lynde's  Physics  of  the  Household,  published 
by  The  Macmillan  Co. 


side  of  the  postcard  and  close  to  it,  and  arrange  two  shades 
to  prevent  the  direct  light  from  falling  on  the  lens.  Hold  a 
postcard,  or  other  object,  against  the  black  end,  focus  the  lens 
on  a  white  screen  about  2'  X  2',  and  your  lantern  is  finished.  The 
illustration  shows  the  lantern  with  the  top  and  one  side  re- 
moved. The  top  should  have  a  trapdoor  at  the  rear  end  through 
which  you  can  insert  and  remove  the  postcards.  The  audience, 
is  seated  on  the  side  of  the  screen  away  from  the  lantern. 

Experiment  No.  124.  Fun  at  night.  You  can  put  on  a  magic- 
lantern  show  with  oil  lamps  or  electric  lights  as  shown  in  Fig. 
182.  The  doorway  between  two  rooms  is  covered  by  two  heavy 
curtains  and  the  8-inch  lens  in  a  ring  holder  is  inserted  in  a 
hole  in  a  piece  of  cardboard  and  pinned  between  the  two  cur- 
tains. A  black  book  stands  10  inches  from  the  lens,  and  is  illu- 
minated by  two  strong 


lamps ;  two  screens 
prevent  the  direct 
light  of  the  lamps 
from  striking  the  lens. 
A  white  tissue  paper 
or  cloth  screen,  2'  X  2', 
is  on  the  opposite  side 
of  the  door  40  inches 
from  the  lens,  the  au- 
dience is  beyond  the 


Screen,  and  if  now  you  Fig.  184.    You  illustrate  the  camera 


GILBERT  LIGHT  EXPERIMENTS 


121 


Fig.  185.   The  camera  obscura 


hold  postcards,  drawings, 
and  other  small  objects 
upside  down  against  the 
book,  the  lens  will  throw 
erect  and  enlarged  images 
on  the  screen,  and  your 
show  is  on. 

The  Photographic 
Camera  is  simply  a  light- 
proof  box  with  a  converg- 
ing lens  in  one  side  and 
a  plate  holder  in  the 
other.  The  lens  L  (Fig. 
183)  throws  an  inverted 
image  ba  of  the  object 
AB  on  the  plate  S. 

Experiment  No.  125. 
To  illustrate  the  camera.  Put  your  converging  lenses  in  turn  in 
a  ring  holder,  and  put  the  holder  in  a  hole  in  one  end  of  a  card- 
board box  (Fig.  184).  Cover  the  box  and  your  head  with  a  dark 
cloth  and  move  the  screen  back  and  forth  until  you  get  a  picture. 

The  Camera 
Obscura(Fig.l85) 
has  a  combined 
lens  and  reflecting 
prism  at  the  top 
which  throws  a 
picture  down  on 
the  table  in  front 
of  the  artist. 

E  x  p  e  r  i  ment 
No.  126.  To  make 

Fig.  186.    You  make  a  camera  obscura  a    Camera  ob- 


122 


GILBERT  BOY  ENGINEERING 


scura.  Arrange  the  8-inch  lens,  mir- 
ror, and  box  as  in  Fig.  186.  Cover  the 
front  of  the  box  and  your  head  with 
a  black  cloth.  Do  you  get  a  beau- 
tiful picture  on  the  white  paper  at 
the  bottom  of  the  box? 

Experiment  No.  127.  A  moving- 
picture  show.  Use  the  camera  ob- 
scura  on  a  table  outdoors  or  near  a 
window  and  let  two  of  you  get  under 
the  black  cloth  and  look  at  the  pic- 
ture, while  two  others  go  through 
funny  antics  outdoors  about  30  feet 
from  the  camera.  Do  those  under 
the  cloth  see  a  very  funny  moving- 
picture  show?  Change  places  and 
repeat. 

Experiment  No.  128.  A  sub- 
marine periscope.  Arrange  the  apparatus  as  in  Fig.  187  with 
the  mirror  at  45°  at  the  top  of  a  long  cardboard  tube  and  observe 
the  paper  under  the  black  cloth.  Do  you  get  a  fine  picture  on 
the  paper? 

This  illustrates  the  construction  of  one  type  of  submarine 
periscope. 

The  Stereoscope  (Fig. 
188)  turns  two  pictures  into 
one  that  stands  out.  The 
glasses  are  prismatic  lenses 
placed  edge  to  edge;  they 
take  light  from  the  two  pic- 
tures AA,  A2B2,  Fig.  189, 
and  diverge  it  so  that  it  ap- 
pears tO  COme  from  One  piC-  Fig.  188,    The  stereoscope 


Fig.  187.  You  make  a  submarine 
periscope 


GILBERT  LIGHT  EXPERIMENTS  123 


tare  AB.  The  pictures  are  taken  in  a  stereo- 
scopic camera,  which  is  simply  two  cameras 
side  by  side  and  a  short  distance  apart. 

Your  Eye  (Fig.  190)  has  an  outer  horny 
membrane  called  the  cornea  and  behind  this 
a  watery  liquid  called  the  aqueous  humor, 
behind  this  a  muscular  lens  called  the 
crystalline  lens  and  inside  this  another  fluid 
called  the  vitreous  humor.  At  the  back  is 
the  nerve  layer,  the  retina,  which  receives  the 
sight  impression,  and  behind  the  retina  is  a 
black  coating  which  shuts  out  all  light  ex- 


189.     How  the 
stereoscope  works 
From    Lynde's  Phys- 
ics of  the  Household, 
published      by  The 
Macmillan  Co. 


tmpl 


optic  aervK 


Fig.  190.    Your  eye 
From  Black  and  Davis'  Practical  Phys- 
ics, published  by  The  Macmillan  Co. 


focused  by  moving  the  lens 
back  and  forth ;  but  the  eye  is 
focused  by  changing  the  shape 
of  the  lens  and,  therefore,  its 
focal  length.  The  muscles  of 
the  eye  make  the  crystalline 
lens  more  convex  when  we  view 
an  object  near  at  hand  and  less 
convex  when  we  view  one  at  a 
distance. 


cept  that  which  comes  through 
the  lens.  The  colored  part  of 
the  eye  is  the  iris  and  the  open- 
ing in  the  iris  is  the  pupil.  The 
iris  contracts  the  size  of  the 
pupil  in  a  strong  light  and  en- 
larges it  in  a  dim  light. 

The  eye  is  very  much  like  a 
camera,  but  there  is  one  strik- 
ing difference:  the  camera  is 


Fig.  191.    The  reason  for  the  use  of 

spectacles 

From  Lynde's  Physics  of  the  Household, 
published  by  The  Macmillan  Co. 


124 


GILBERT  BOY  ENGINEERING 


Spectacles.  The  eyes  of  short-sighted  people  focus  the  light 
in  front  of  the  retina  F,  Fig.  191  A,  and  this  difficulty  is  overcome 
by  spectacles  with  diverging  lenses,  L. 

The  eyes  of  long-sighted  people  focus  behind  the  retina  F, 
Fig.  191  B,  and  this  difficulty  is  corrected  by  spectacles  with 
converging  lenses,  L. 

Experiment  No.  129.  To  look  through  your  hand.  Your  two 
eyes  look  along  converging  lines  when  you  look  at  any  object, 


Fig.  192.    To  make  the  bird  enter  the  cage 


and  this  leads  to  the  following  apparent  magic.  Roll  a  piece  of 
paper  into  a  tube,  hold  it  beside  your  hand,  look  at  your  hand 
with  one  eye  and  through' the  tube  with  the  other.  Do  you 
appear  to  see  through  your  hand?  Look  through  other  things 
in  this  way. 

Experiment  No.  130.  To  put  the  bird  into  the  cage.  Draw 
a  cage  and  a  bird  with  centers  about  2  inches  apart  on  paper, 
stand  a  card  on  the  line  AB  between  them  (Fig.  192),  then  look  at 


GILBERT  LIGHT  EXPERIMENTS  125 


the  cage  with  one  eye  and  at  the  bird  with  the  other.  Does  the 
bird  enter  the  cage  ? 

The  Moving-Picture  Machine  (Fig.  193)  throws  12  to  16 
pictures  on  the  screen  each  second  and  shuts  off  the  light  while 
one  picture  is  changing  to  the  next.  The  pictures  are  taken  at 
the  same  intervals  and  differ  very  slightly  one  from  the  next 
(Fig.  194). 


Fig.  193,   A  moving-picture  machine 


The  "Why"  of  the  Movies.  The  reason  you  see  the  pictures 
continuously  and  are  not  aware  that  the  light  has  been  shut 
off  is  that  your  eyes  retain  each  picture  for  a  short  time  after 
it  has  left  the  screen.    You  will  now  illustrate  this. 

Experiment  No.  131.  Circles  of  fire.  Go  into  a  dark  room, 
light  a  match,  blow  it  out  but  keep  the  live  coal,  and  then  wave 


126 


GILBERT  BOY  ENGINEERING 


it  in  the  air.  Do  you  see  circles  of  fire?  You  do,  because  your 
eye  retains  the  impressions  for  some  time. 

Experiment  No.  132.  To  put  the  bird  into  the  cage.  Draw 
a  bird  on  one  side  of  a  piece  of  cardboard  and  a  cage  exactly 
opposite  on  the  other  side.  Attach  cords  above  and  below  and 
spin  the  cardboard.  Does  the  bird  appear  to  enter  the  cage? 
It  does,  because  your  eyes  retain  the  pictures  of  the  cage 
and  bird  for  a  short  time. 


Fig.  194.    The  "why"  of  the  monies 


In  the  Dark ! 

A  knock  on  the  head  with 
a  hatchet  or  a  stab  with  a 
knife  doesn't  sound  pleasant, 
but  you'll  enjoy  apparent 
treatment  of  this  kind  and 
so  will  your  friends  who 
watch  your  shadow  show. 
Make  your  boy  friend  rise 
in  the  air— change  him  into 
a  bird  or  a  cat — create 
freakish  images.  It's  easy ! 
And  laugh —your  audience 
sure  will  enjoy  it  because  it's 
new— nothing  like  it.  An 
entertainment  made  for 
boys  who  want  real  fun. 
But  that's  only  a  few  of  the 
many  things  you  can  do  with 

GILBERT 
LIGHT  EXPERIMENTS 

One  of  these  outfits  will  help  you  to  understand  a  great  many  facts  about  light. 
You  can  perform  a  number  of  experiments  which  explain  the  laws  of  light. 
Learn  about  the  movie  machine,  the  telescope  and  other  optical  instruments. 
There's  a  big  book  on  Light  with  each  set,  it's  a  handy  size,  just  right  to  put  in 
your  pocket 

From  this  book  and  your  set  you'll  get  a  knowledge  of  light  that  will  be  helpful 
to  you  always.  It's  great  fun  too,  the  kind  you  like.  The  outfit  is  complete  with 
prisms,  mirrors  and  all  the  apparatus  you'll  need  to  perform  the  experiments. 

Ask  your  dealer  to  show  you  this  new  Gilbert  toy. 
If  he  hasn't  it  write 

THE  A.  C.  GILBERT  COMPANY 

507  Blatchley  Ave.,  New  Haven,  Conn. 

In  Canada  —  The  A.  C.  Gilbert-Menzies  Co.,  Limited,  Toronto,  Ont. 

In  England  — The  A.  C.  Gilbert  Co.,  125  High  Holborn,  London,  W.  C.  1 


The  Greatest 
Book  for  Boys 
in  Years 


BOY  ENGINEERING 


The  Most  Helpful 
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Published 


Think  of  it!  "Football  Strategy,"  by  Walter 
Camp— "How  to  Pole  Vault,"  by  Former 
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by   Eddie   Rickenbacker,    and    "  Athletic 
Training,"  by   the  famous  Yale  trainer, 
Johnny  Mack.    Chapters  about  signalling, 
wireless,    wonderful   heat,   sound  and 
light  experiments,  how  to  build  a  real 
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and  pneumatic  engineering  and  surveying, 
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worth  dollars  to  you. 

Buy  it  from  your  dealer,  or 
send  us  25c  to-day.  You'll 
never  be  sorry 

The  A.C.  Gilbert 
Company 

507  Blatchley  Avenue 
New  Haven   :  Conn. 


GETTY  CENTER  LIBRARY 


3  3125  00019  0385 


